I (m18) was at my friends 19th birthday party with my girlfriend (f18). We were down at the local campground, camping for the night.
My girlfriend and I were sleeping in the back of my car. We were laying together, her body back up against mine, when I got an erection. It pressed up against her ass, and I heard her gasp a little when I pushed into her. We started doing foreplay, kissing and rubbing and all that, and I pulled her pants down alongside mine, continuing foreplay with my cock pressed against her.
Eventually, I flipped her onto her stomach and started pressing up on top of her. Kissing her back and neck, rubbing my hands along her body. I had my cock between her butt cheeks, when I noticed she would gasp a little harder when my cock head got close to her ass. Just to test the waters, I moved the head up and down, pressing into her asshole, and sure enough, she liked it.
I got one of my fingers wet, started fingering her, adding more as I went, until eventually she let me put my cock in her ass.
The whole thing felt so surreal, I was going to talk to her about it eventually but I wanted to wait until later, but I ended up finding out accidentally and I can't believe she likes it like I do.