What do men vs women think about during sex?

I've heard that men are more likely to be thinking about other people during sex and that women are more likely to be thinking about their partner. What do you think? Have you ever asked your partner if that's true?

I personally do think about my partner and am in the moment. It kind of sucks to know he might be thinking about someone else. :/ Especially since he's going to strip clubs this weekend for a bachelor party.