When you take this to the extreme, it's like, "men should enjoy another man's penis as much as women do if it wasn't for those damn external societal pressures." It's like... there can't ever be biological differences or biological reasons for any kind of preference at all?

It doesn't add up. If you acknowledge men and women are different then you have to acknowledge there are biological reasons for why some people might prefer some other things than others. If you don't acknowledge any differences then you must not believe sexual orientation is real - in which case, you're fucking crazy.