Try to smooth talk her

You've taken things this far already, and decide you're "all in". Arms out to your side in a nonthreatening manner, you slowly approach her. "I'm sorry, Marissa, I got carried away. But I'm serious, I find you very attractive.". She doesn't back away, and by now, you're close enough to place your hands back on her shoulders with her holding your gaze this time. "Any man would be lucky to be with you, and I want to be that man.". You lean towards her and lightly kiss her on her lips. Your lips linger on each others and you feel her relax in your grasp. Sliding your hands up her neck to the sides of her head, you pull her in for a deeper kiss, darting your tongue into her mouth. Her tongue finds yours and you feel her drop her sweater to the ground, moving her hands from between you to the back of your shoulders. Moving your mouth from hers, you lightly kiss her chin, up along her jawline, to her ear. You take her lobe into your mouth, nibbling on it between your teeth.

Her shallow breaths quicken. Whispering in your ear she says "David, we can't do this.". But makes no move to stop you.

"Yes we can" you whisper back. "I can tell you want this just as much as I do."

"But your girlfriend..."

"She's not important" you interrupt. "You're what's important. I want you, not her. I need you Marissa.". Pulling her back in for a deep kiss, you rub your pelvis against her, letting her feel your fully engorged member through your pants. You slide your hands down to her plump ass cheeks, squeezing them, and pushing her hips against yours.

Hearing you reject your girlfriend for her has softened Marissa's resistance. Her tongue is twirling circles around yours and her chest is heaving as she becomes more excited by the events taking place. She had always thought you were out of her league and would never be more than just friends, but had secretly lusted after you since freshman year of high school. Now that those dreams were becoming reality, she wasn't going to let this moment pass. "I want you too, David. I want you to take me. I've wanted it for years."

Seizing the opportunity, you grasp the bottom of her t-shirt and slide it up over her head, Marissa raising her arms in the air allowing you to do so, revealing a sensible, white, cotton bra. The two of you resume kissing and begin shuffling towards your bedroom. Marissa begins fumbling with your pants, trying to unfasten them as you go, while you work on the clasp of her bra. You feel the clasp spring free at the same time that she manages to free your button and unzip your pants. Pausing in the doorway to your room, you slide the straps of her bra off her shoulders letting it drop to the ground between you. Marissa reflexively moves to cover herself with her hands, but you stop her before she can. "Please don't cover yourself" you ask gently. "They're beautiful. You're beautiful.". She lowers her hands, giving you a view of her pale, white, freckled, breasts. Leaning down, you cup her breast; feeling her D-cup fill your hand, and raise her up-turned, pale, pink, nipple to your mouth. You flick your tongue around her small areola, illiciting gasps of surprise from Marissa. Suckling on her nipple further quickens her breathing and she begins running her fingers through your hair. You manage to back her further into your room, directing her towards your bed that she falls onto once the back of her knees bump into the edge of the mattress. She props herself onto her elbows to watch as you undress yourself in front of her. Completely naked, you start your work on her; removing her shoes, socks, then unbuttoning her pants and sliding them off, leaving her in nothing but a pair of powder blue, cotton, bikini underwear. Wasting no time, you slide those off next, Marissa lifting her hips off the bed to help you, revealing her full, hairy, bush. She's trimmed the boarders so the hair doesn't spill out the side of her underwear, but has otherwise left it untouched.