Same reason that women do this, because men can be men around other men in a way that they usually aren't when a woman is around. They can generally make more offensive jokes without a woman getting offended, they can take more risks without worrying as much. Bring a woman into this and the dynamic changes, especially if romance is involved. Same when it comes to women being able to be women around other women, there are some things they wouldn't want to talk about or do with a man around because it isn't polite and the group becomes more uncomfortable.