What really would you call a first kiss? If you considered the loving pecks that her parents had given her throughout the years, she had a handful. Forehead kisses and cheek kisses and over the top affectionate kisses when they were both laughing so hard that it basically made no difference.
Or you could count the kisses that she gave her parents, usually just kisses on the cheeks, when she was feeling particularly affectionate, an 'I love you' without the need to say it out loud but usually followed by one or something similar.
You could even count every kiss that she'd received since becoming Ladybug, every single hand kiss that Cat Noir gave her, especially the first time that he bent down, holding her hand in his, and gave it a gentle kiss. Ladybug would have to count the kisses that she gave too.
That day against Glaciator as she leaned forward and kissed Cat Noir's cheek or even the kiss that he gave her in return that evening or even when she bent down over a coffin, knowing that Adrien was hiding in it as she'd just had him get into it, and when she kissed the pharoah's lips right where his would be a little farther underneath it, heart nervous, but wishing that she could just touch him.
You'd have to count the kiss on the lips that she'd used on her partner to save him from Dark Cupid's spell even though it felt far more platonic than Ladybug wanted to consider her first kiss to really be. It meant something from the way that her heart slowed in nerves during the kiss.
Marinette would count the first forehead kiss that Adrien gave her as a first kiss of hers too, when she was in his arms and dancing slowly, and he felt the need to affectionately keep her close with a motion so sweet and tender. She still wasn't sure that that wasn't a dream or if she'd just imagined that moment.
This though seemed so much different than a hundred, million 'first' kisses as something real was hung up in the air here as Adrien leaned down and kissed her earnestly, as her heart raced, and her mind couldn't even think to wander away from this moment. It was soft, slow, and sweet. Marinette was positive that it was the most tender thing that Adrien could have ever done, like finally getting that kiss that she sort of really wanted that day when they were just playing a part for school, and it was way better than she imagined.
She almost kind of knew how he might move based off of true love's kiss with Cat Noir, and so she'd kind of expected something like this as she tried to pour her whole heart into the kiss too as Adrien cradled her head close and as she felt her heart race and jump in time with his. Each second was like perfected bliss even though neither really knew what to do other than move their lips together as she pressed closer.
Each second felt so tempting and sweet as she leaned against him, pressing steadily against him, moving gently with the pace that he set. She was too scared to even think of deepening it when it felt so nice to move her lips along with his and just focus on every soft, static energy between them flourish. It was everything to her even though she'd only had it once when he pulled away just a second later, a second too soon, even as she caught her breath and just stared at him.
'I love you,' was on the tip of her tongue, but it seemed stupid to say it after just one kiss since she'd heard how new and almost 'scary' that could be for new relationships and yet she just knew that with a few more kisses she might not be able to hold the words back. Marinette smiled as she gazed into her new boyfriend's eyes, and she just hoped that he'd give her many more kisses to try and figure out what they're doing together and a million more chances to be near him, to really discover what 'I love you,' meant, because every second together should teach her more of what it meant.
Marinette wanted all of her first kisses to be with him now even though she knew that she'd only ever had that one if what most people said was true. She didn't want a first kiss with anyone else ever, just Adrien. She'd make an exception for her future kids that would hopefully be his too one day since they really would need kisses to help 'boo-boos' heal and to have goodnight kisses before bed.
She'd gladly shower them in kisses, love, and affection as long as they'd let her or as long as she can catch another moment or two to give them kisses. Either way, she was positive that she wasn't ready for that yet as she and Adrien were just starting to date and not married.
Marinette would have years to give as many 'first' kisses to Adrien as she could. She'd never given him a forehead kiss yet or a kiss on the cheek or anything liket hat yet, and so she still had some 'first' kisses left to give him and hopefully all the years that she could to give him them.