I hope I'm not rambling, here, but I kind of know what you're talking about, so maybe it will help:

I'm not all that into anal sex. It hurts sometimes (once he pushes in past a certain point, it's too thick - but he's good about just partial penetration), and other times anal play is just neutral for me. Not positive or negative; I don't get pleasure from it but just the head isn't painful either, so he plays around to his heart's content.

I know how much my boyfriend likes it. HIS pleasure of it makes me go wild, so I don't mind doing it. He makes me feel safe and loved and secure and I don't mind doing it for him. He lets me decide when, but occasionally I'm like "Why don't we....." even though I don't really get anything out of it.