can men cum without realizing it?

i'm going to try to keep this as clean as possible: so i was having butt sex with my bf but we had to stop cause i had to leave for work so he said he didn't finish cause he would have known if he did. but later when i got home i was checking the damage he did to me, and i saw cum come out of my butt. and i know what cum looks like, clearish whiteish. i know it wasn't lube, i know it wasn't my own cum because mine looks different than that. i know it wasn't precum bc there was too much of it and precum is clear. he insists he didn't cum but i definately had cum in me. so that is why i am curious.