I understand exactly where you are coming from, and what you mean.
Okay if someone creates an rp, they make their rules for it, and if the rules aren't to your taste, then you don't play, but they can still be offensive surely to some people, it's like posting offensive posts, and saying don't read them if it is offensive! If they are in a public place you have the right to be offended and criticise.
So I think the idea of setting a rule like that in general is rather wrong...
Unless it is in context of the story, or for gameplay purposes...such as

If it suits the storyline better. You probably would have openly public gay relationship in Victorian London for example.

Or if the number of male female characters call for it, and somehow homosexual relationships effect the balance of the characters interaction? I'm sure other people would have better examples or reasons why, but it can't really think of any particularly good ones?

I like to think when role playing we are creating stories, and playing a character, I see it as a matter of skill to play or write a character,that I either have nothing in common with, despise, have different views from or even different gender or sexuality. So to put limits on that I feel speaks about the person creating the rules don't you?