Your best friend gets cancelled on before her date with a fwb. She is upset not having been fucked in MONTHS, so you offer to fuck her instead. At first, she laughs, saying you're like a step-bro to her. She says she would only fuck you if you just laid there and let her use you. It turns you on to even hear her say that, and she surprisingly takes you up on it. Kissing you and laying you down on the couch, she mounts you in cowgirl, grinding her wet cunt on your hard cock. Finally, she slips it in, grinding and riding you in cowgirl and reverse cowgirl until you fill her pussy with cum. Even then, she keeps going, overstimulating your cock after your orgasm until she cums as well... Don't expect this to happen with your best friend often!