Aged-up, roughly early 20s Starco here. Long time dating, not yet engaged.
The magic rainbow dolphins made a good case, but it was the beachside service from the sentient puppy waiter that sealed the deal.
This was their favorite dimension on their "Super Spring Break Bang-Bang Blowout Inter-Dimensional Tour!" (Star insisted she could hear when Marco left off an exclamation point).
"Your parents are really really sure, Star? Are you sure you heard them right?"
"Uh-HUUUUUUUUUHHHHH, Marco. For the last time. They'll let us stay in the Royal Suites, together, in each of the Grand Inter-Dimensional Resorts. They said it was for 'good behavior'! And something about 'grooming' you, ugh. I don't even want to know what they're thinking there..."
"I...I guess that makes sense,"he admitted.
"Hmmmmmm? I don't know..."
Star sauntered over, the sway in her hips and the wicked twinkle in her eye betraying the innocent smile.
"...you would be the one to know all about my naughty moments, wouldn't you, Wild Man?"
Marco was putty in princess arms. Her face hovered close enough that he could already taste her.
"I, uh, might know a thing or two."
He closed the gap and kissed her deep enough to keep her speechless another few seconds.
"But don't worry, Princess. Your secrets are safe with me, you know I'll never talk."
"That's because," she mewled against his lips, "you have more important things to do with that mouth instead..."
Marco was never one to let important business slide. Ever diligent, especially when it came to taking care of his darling would-be queen.
And so their Spring Break plans were set. Neither could resist the excitement of exotic locales, and the lure of luxury accommodations didn't hurt. Certainly, the sights had been worth missing out on just another undergrad romp in South Beach, or sleeping in at the Diaz home.
They'd danced with Princess Pony Head (also out on "good behavior"), they'd skied the slopes of Mount Elsa, they'd bungee jumped from the clouds of Olympia, and they'd ridden dragons through the lush forests of Olgrimore.
But they agreed: nothing could compare to Lisanfrak.
Bright, technicolor animals at your beck and call, rainbow everything, and a veritable beach paradise. They'd spent most of the time running around and swimming, racing dolphins near the shore. Marco would eventually tire and head back for the beach blanket. Still, lounging back and admiring his girlfriend frolic in a bikini in the middle of a kind of parasise was more than satisfying.
"Your drink, sir Mr. Marco," barked the puppy waiter, wagging its tail expectantly.
"Thank you very much, buddy."
He scratched the pup behind the ears, and it scampered off happily. Local tipping custom, always good to know.
He was too relaxed to head back to the shoreline. He needed the relaxation, the more he thought about it. This vacation was starting to eat at him,and he wasn't sure he could properly explain to her...
"Marco! Come on, we need one more player!"
Some kind of cross between a chicken fight and a game of water polo had broken out. Marco could hardly resist her puppy dog face, certainly not when it was glistening in the odd, pink-tinged rainbow sunlight of this dimension. The hearts on her cheeks shone like beacons, luring him in towards her. He rose and plodded his way over to the shoreline.
"Hey you, why so grumpy?"
"I'm not grumpy at all Star, let's do this."
She wasn't falling for his half-hearted attempt to dodge her questions.
"Spill it, m''Daring Diaz', what's gotten into you? Are you really that bored with your vacation already?"
"No, not at all Star!" he quickly added. He took her hands in his and shared a tender glance. "This has been the most incredible vacation, with the most incredible company..."
He took a deep breath. Star nodded to the dolphins to start without them, then returned her attention to Marco. She reached up to stroke his face soothingly, as she'd done so many times before over the years when he became overwhelmed with the world beyond his dimension.
"I just...I get worried, sometimes."
"I hadn't noticed," she said, sticking out her tongue and giggling softly.
"Yeah yeah, but I mean it. All the adventures, the fighting monsters...it scares me sometimes. Not so much the fighting, I know what we can do together as a team..."
They exchanged a warm glance, as he continued.
"..but the other stuff. The things that seem ordinary to you, the trans-dimensional craziness, even after the last few years I still don't know what I don't know, and I don't know when something might hurt you."
"I think I can take care of myself, Marco."
"I know that!"
"Then what are you saying?"
"I'm saying that...it's never going to be easy, you and me."
At the sight of her heart nearly breaking on the spot, he wrapped his arms around her tightly and whispered soothing assurances that he hadn't meant what she surely must have thought.
"I just mean...I'm always going to be human."
"A very strong and very brave human."
"But I won't always be that either, Star," he pulled back, "you'll always be a magical princess with your amazing powers...and I can barely keep up with you now. It's not easy, feeling like...like I can't protect you. Or help you. Or maybe even one day go on these adventure vacations with you. And I guess...this vacation has been reminding me of that somehow. That even the fun times are going to be...hard."
She had let him speak, barely containing the urge to spill her own guts, huge watery eyes hinting at what might come when he stopped. She'd grown much more patient since when they first met.
"...and you know. I know there's a lot of other guys from Mewni and all over the dimensions, who would make things easier and who would love-"
"Don't you even finish that thought, Marco Diaz."
She rose to her tippy toes in a futile effort to stare him down, but Marco got the effect. She looked angry, ready to strike.
Which was convenient, as the glass from the hotel and sounds of an explosion nearby rang out. Ludo's goons were in need of their version of a good time at the beach.
"Kill them, all of them, you fools!"
Ludo charged forward, lightning shooting from his fingertips. Star raised a barrier to keep them protected.
"Remind me to put you in your place as soon as we're done here, dear."
Marco gulped. "Yes m'am."
They sprang into action with a well practiced routine. Star's magic against the flying creatures, and the quicker ones, Marco's fists against the slower, larger ones. She'd grown much more powerful since their first meeting, and she even drew upon the magical strength of the tecnhicolor animals who'd' been scattered along the beach. Marco's swift kicks and knife hands were appreciated, but Star was a sight to behold.
With all but Ludo to defeat, Marco circled Star's old nemesis, waiting for his opening.
"I could devour you, boy," Ludo snarled.
"Then come and get me," Marco quipped back.
The light from the wand glinted off the sand. Star had risen into the air, twirling with magical beams surrounding her as she began to prepare a spell that Ludo would never defend. Not with Marco right there, ready to stop any counter he'd had left. Ludo growled and tore open a dimensional portal.
"Just one more battle, Star Butterfly! The war is yet to be decided!"
He leaped through the portal, with his minions scrambling back in after him. Star landed upon the beach, abandoning the spell and making her way back to Marco.
"Star! You were...amazing! Like always!"
Her unusually stern stare told him this wouldn't be like their usual post-battle adorations...
"Oh, I know." She stopped not even an inch in front of him, but Marco wasn't feeling the usual urge to close the distance. "And you weren't so bad yourself, Karate Kid. But I did cast some preeeeety awesome spells, and I miiiiight have been able to take care of those guys myself..."
Marco's heart drooped. Star looked down and to the side, curling her toes in the sand as she held her face pensively. Her eyes glanced back toward Marco, and she smirked.
"And do you know why my magic is so strong? Huh? Do ya?"
Marco shook his head.
"No? Never wondered once, why I got so much better since coming to Earth?"
Star sighed adorably, like a kitten yawning near her favorite human, relaxed and in control.
"Marco, it's because of you."
"Wha-what? How-"
"Like you said, I'm a magical princess. This part the stories on Earth almost got right: a huge part of my magic comes from love. And you-"
She jabbed a well-manicured finger into his deceptively muscled chest.
"-are still a big part of that." Her voice grew tiny. "And that's why I don't want you to even think for a second that I should move on to some other guy from Mewni, or that you're holding me back, or that you need to worry about me when you can't beat up every danger that might come my-our way."
She dragged her finger tip up to his face, tickling his mouth and tapping his nose.
"You're stuck with me, hot shot. Because I love you, and you make me able to do the hard stuff. And if you get old, or hurt, or too tired or too scardy to 'keep up', I'll slow down just enough to pick you back up. It won't be easy, I'm never going to be an 'afternoon tea' kind of princess," they both chuckled, "but that's our life, get used to it. I'll always be here, and I'll always trust you, and we'll always have time to just enjoy each other in between the 'fairy tales'...that is, if you want...and you'll still have me even with all this..."
Marco didn't give her time to seriously think his answer could be any different than the first time she made absolutely sure he could stomach dating a magical princess, several years ago. He swept her up in his arms and kissed her deeply, eliciting the most satisfying moan from his demure-only-in-appearance sweetheart.
"Of course I still want to be by your side, Star. No matter what I can or can't do-"
"You do a lot just being a good kisser..."
"Well," he bent down and kissed her again, both of them laughing in between pecks on the lips, "there is that."
They nuzzled each other softly. The dolphins had just about finished their game in the water. The couple held each other close.
"I love you too, Star. And I feel better now. Whatever happens...I'll be there for you however I still can. But..."
"But what, now?"
"...I just want to spend some time relaxing with you, we've been so busy this vacation."
He slid one hand down her spine, and stroked the other toward her midriff.
"I feel like we still need to relax..."
He leaned his head in. Star's breathing began to quicken.
"Appreciate the royal suite while we still have a chance..."
He nibbled on her ear, and she whimpered as she pressed herself close.
"Spend some quality time together..."
He placed a kiss on her neck, drawing a quiet, breathy moan from her. She clutched at his back.
"Because we are stuck with each other, aren't we?"
Star kissed him hard and passionate before pulling back and kissing him again, tender and lovingly.
"You have no idea..."
She turned her head to whisper in his ear.
"...but you might be about to find out."
They giggled playfully the entire run back to the room, Marco chasing her all the way, never far behind. The dolphins chittered away in the water, amused at yet another silly young couple madly in love.
Best dimension on the vacation, by far, they'd agree when they returned.