Well. Just picture the opposite. This is a human thing. Not a guy thing. How do you feel if I spend our first date talking about all the bad (and GOOD!) things my ex-girlfriends did?
Thing is, we all know we likely won't be the one for someone else. But we do so enjoy to entertain the notion that we could be. Knowing about other people makes that pretense more challenging. And we're all ever so fragile little beings.