Then you fervently hope he stays inside until he has finished twitching, and gone soft. If you're in the traditional woman's position - on your back, thighs wide, legs up - you can wrap legs around and try to hold him inside until you have been inseminated.
Did you talk about how it felt inside you? My second time with a guy (he was much older) he asked me all sorts of questions while he was still inside of me, post orgasm. Don't remember them all, but it felt really intimate with him on top, his penis and cum inside my ass and just cuddling and making out, looking into each other's eyes.
The guy who got my ass cherry stayed inside, still erect, kissing me, after he finished twitching. He made a few long, slow, strokes. I could tell that the texture had changed inside me, and knew that his cum was spread all up and down my rectum.