Okay, so I was having a discussion with my boyfriend and he said something that made me go "What the...?!" in my head. He told me that sometimes when guys are having sex, they just know it's just "not going to happen". He said it's a hard concept for some girls to accept/grasp which leads to some guys faking it. I know it's possible to fake it, for guys, that's not what i'm questioning here. I just am wondering, is there just some times where you can't cum during intercourse? If so, how do you tell the girl "yeah, it's just not going to happen this time."