Disclaimer: I don't own Phineas and Ferb, credit goes to Dan Povenmire and Jeff "Swampy" Marsh.

This is a re-post because last time I rated it T and I got only flames, so I wanted to see if people would actually like it if it was M.

Before you read this, you should know that it contains mature content, so that's why I rated it M this time, to avoid getting me 50 million flames like before, so here it is.

Phineas steeled himself for what he was about to do. He had been thinking long and hard about it for weeks, and had finally made up his mind. He took several deep breaths, calming himself, and then knocked on the door.

Phineas and Isabella had been living together for six months. As soon as they both turned 18, they had moved into a small one-bedroom apartment. They had been a couple for five years, after Isabella had finally confessed her feelings for Phineas at his 13th birthday party. He had been overjoyed, realizing for the first time that he loved her as well. They had stayed together through high school, and were finally ready to move in together shortly before graduation. Until a few weeks ago, neither of them had really been ready to take the next step in their relationship, but now Phineas was sure that this was what he wanted.

Isabella heard the knock on the bedroom door, knowing it was Phineas, because there was no one visiting at the moment. They had just finished watching a movie together, and Isabella had excused herself to the bedroom to change to get ready for bed. He fully well knew that, so that was why he had knocked. Isabella looked down to assess herself quickly. She was still wearing her jean shorts from the day, and on top she had only her bra on. It's not like Phineas had never seen her in it before, so she smiled slightly before answering.

"Come on in, Phineas," she said. After a second the door opened, and Phineas walked in, looking slightly embarrassed when he saw her. Isabella blushed, but did nothing to cover herself.

"What's up?" she asked, highly curious now. If it was so important that he needed to come in now, then she had a right to be curious. Phineas seemed to prepare himself before walking slowly towards her.

"There's just something, that I've been wanting to do for a while now..." he said, trailing off and looking down. Isabella still didn't know what he was talking about, so she nodded.

"What is it Phineas?" she asked, completely unprepared for what happened next. Phineas hesitated a second, and then took a step forward, closing the gap between them and kissing her. Isabella kissed him back enthusiastically, still not aware of what was about to happen. Normally, Phineas would pull away after a few seconds, but this time he deepened the kiss, pulling her towards him. Isabella put her arms around his neck, surprised and pleased with his reaction. They continued this way for several seconds, and then Phineas put his arms around her as well, slowly moving his hands down her back to just above her waist, testing her reaction. She seemed to react to it, kissing him more passionately then before, so Phineas slowly moved his hands around to her hips, which only pleased her more. Phineas knew that there was no turning back now, so he moved his hands around to her front on her waist line, starting to undo the button on the front of her shorts. Isabella finally realized what he was doing, and she pulled her lips away from his for a moment, resting her forehead on his.

"Phineas," she said, slightly breathless, "are you sure you want to do this now?"

Phineas didn't hesitate for a second answering her.

"Yes, I'm totally sure," was his answer, and so Isabella pulled him in again to continue the kiss. Phineas undid the button and carefully slid her shorts down, revealing her black lace underwear. Her breathe quickened as he rubbed his hands over her back and sides, and she pulled him back, making them both fall back on the bed with Phineas lying on top of her. She let her instincts take over as she deepened the kiss even more. This was about as far as they had ever gone before. Isabella smiled playfully, twisting them around so that she was on top. She brought herself up and straddled him, starting to undo the buttons on the front of his shirt. When they were undone she pulled his shirt off, tossing it carelessly aside. Phineas smirked and pulled her back down, flipping around again so he was on top. He carefully slid his hands around her back, undoing the clip on her bra. Isabella slid her arms out and Phineas tossed it aside to the floor.

Their breathing quickened, neither of them used to this feeling, but that didn't mean they didn't like it. Phineas started with his hands near her face, slowly moving them down, his fingers tracing her soft skin, and bringing them to a stop on her chest. Isabella's heart beat wildly, which Phineas felt as he was still on top of her. Their kiss became desperate, as if they needed each other to survive. Isabella hands moved down Phineas, making their way to his waist, where she undid the button on his jeans, slowly sliding them down. Phineas kicked them off, revelling from the new feelings he was experiencing. He lightly squeezed his hands, making Isabella moan in delight. Now there was barely anything separating the two of them. This was it, and they both knew it. Phineas pulled away for a second.

"Are you ready to do this, Isabella?" he asked breathlessly. Isabella smiled and didn't hesitate before answering.

"I was born ready, Phineas," she said, and she pulled him back to her, kissing him like there was no tomorrow.

In that single moment, both of them knew. They were truly meant to be together. After years of Isabella desperately trying to get his attention, he had finally understood. Five years later, they were ready to do something that would bind them together forever. Isabella had always known that Phineas was shy, so she was patient, letting him take things at a comfortable speed, never pushing him farther than he was ready to go. Never had she expected that he would be the one to suggest going this far. The fact that he had made her feel like the luckiest girl in the entire world. There was nothing stopping them now.

Isabella slid her hands down him again, playing with the waist-line of his boxer shorts. He reacted by squeezing his hands harder, making her moan in pleasure once again. She slowly started sliding his shorts down, her breath quickening. When they got to his knees Phineas kicked them off, squeezing her breasts again. Then his hands made their way down across her stomach to her underwear, which he didn't hesitate to slide off. He brought his hands back to her chest, and she traced her fingers lightly over his waist, in an almost teasing manner. After a few more seconds, Phineas brought his hand down to clasp Isabella's. They moved as one, kissing each other like they needed to in order to survive. To them, it felt as if that was true. And then Phineas took the final step, connecting them in more ways than one.

Isabella literally stopped breathing. Never before had she experienced a feeling so amazing. She squeezed her eyes shut and kissed him harder. Phineas brought his hands back up to her chest, squeezing yet again, and Isabella had to try not to scream out in pleasure. A small squeak escaped her lips, and Phineas pulled his head back, looking concerned.

"Are you okay?" he asked. "Do you need me to stop?"

Isabella opened her eyes, finally starting to breathe. She smirked as best as she could in the situation.

"No, don't stop," she managed to get out, so Phineas smiled and kissed her again. Isabella did feel some pain, but it felt so good at the same time that she didn't care. She could feel it building, reaching its climax. Phineas started to move faster, not wanting to hurt her, but loving the feeling none the less. He was glad that she wanted to continue. He could hear her moaning, and knew that it meant she liked it.

"Oh, Phineas," moaned Isabella between kisses. She felt him moving faster and faster, and she felt herself beginning to reach the peak. Her breathe was coming in gasps now, but she didn't care. She loved the breathless feeling; it helped complete the experience. Phineas continued to move his hands over her body, quickly finding the spots that got the most reaction out of her. Isabella currently had hers hands just above his waist, where she knew he loved it when she rubbed there. The two of them worked together to make the experience even better for the other.

Finally, after what seemed like forever to her, Isabella gasped as she felt herself climax. She hadn't thought that it could get any better, but that was before she felt this. Nothing compared to it- well, except for Phineas being there with her. Words failed to describe the feeling, as she felt they wouldn't do it justice. Phineas sensed what was happening, as he was reaching the climax himself. He continued to go faster and felt the feeling spread over him. The two of them were panting now, but they continued the kiss desperately. Finally, Phineas began slowing down as the feeling settled down for the both of them. Isabella wrapped her arms around him, and when Phineas pulled out of her, she pulled him close to her, not wanting this amazing experience to end. Phineas hugged her back, and the two of them continued to kiss with the same amount of passion until they were both too tired to anything but lie there together.

Isabella now rested her head on Phineas' chest, while he absently stroked her hair. Neither of them said anything, not needing to after what just happened. They continued to lie this way until the two of them slowly drifted off to sleep. That night, they dreamed about each other, as they always did, but this time it was different. They had done something that had joined them together forever. And it didn't matter that they weren't married yet (although Phineas was going to take care of that relatively soon), and it didn't matter that they were still young. They shared a bond so strong that all that mattered was that they had each other, and that they always would.

The End

I hope that you liked it! I had a sudden burst of inspiration and I just needed to write it down. I was toying with the idea for a long time, and then I finally typed it out and got this. Thanks for reading, and let me know what you thought!