Why is it such a bad thing for a man to fantasize about being with another man?
So what.
Sex feels good no matter who you are doing it with
Fantasies are natural, and enjoyable.
Im not saying that every man needs to think about being with another man. For some people gay/bi fantasies just isnt their cup of tea.
But I dont see anything wrong with it.
People are beautiful, wether man OR woman. We are supposed to be attractive to each other and if you find someone attractive that makes your loins itch, more power to ya.