I've been reading all these great stories and wanted to add my own true story.
I was a somewhat late bloomer and didn't have my first GF until late in college. We were both virgins and the sex was pretty good, at least I thought it was for two people with zero experience. I had always been interested in anal sex and would occasionally ask but never got anywhere, not even a finger in her butt.
We were coming up on a year into our relationship and the week before our anniversary, we were hanging out in her apartment and having a some afternoon sex. Out of the blue, she asks me if I want to put it in her butt, as a sort of early anniversary present. Of course I said yes and my cock was super stiff at this point. We went back to her room and she started off with a nice wet blowjob. She wasn't a fan of receiving oral (never really understood this) so it was straight to pussy for some fucking in missionary. After we both got worked up, she turned around and got on all fours with her butt in the air. She asked if I was ready and the only confirmation she needed was seeing how hard my dick was. I couldn't believe this was going to happen, I just nodded yes.