In case you didn't know or have forgotten, I occasionally write sex articles for the sex toy company Kiiroo. A few months ago however, they asked me for an erotic short story. The one I gave was actually a true story. Recently though, I pitched to them a story that was actual fiction.
The idea was born out of 2 facts I've confessed to when porn is part of my masturbation:
1) I watch more straight porn than gay porn, and;
2) While I'm totally versatile in action, in my fantasies however, I'm more of a bottom.
Keeping those 2 facts in mind, then you are right to conclude that the pleasure one assumes that male is giving that female's vagina is pleasure I'm imagining him giving to my ass.