While I certainly agree, there are times when ditching someone because you don't want to be with them anymore is pretty trashy and you should feel bad about it.

Like if you married someone and promised to be with them through better or worse, and then throw them to the curb when they develop breast cancer and have to have a mastectomy because you "really like breasts." (oddly specific because it happened to my parents' friend).

However, without a doubt, this is not one of those cases.They are dating, and it turns out the person he is dating is a man, and he is a heterosexual male. There is nothing wrong with this being not what he wants and leaving the situation. Although I would recommend leaving on good terms because it still might be nice to have the OP as a friend through this challenging time for them.