See coreyes, I think the issue with MTP and JT is that they are not very good actors.
Someone used the example of Barney on How I Met You Mother, and that is the perfect example. He is such a fantastic actor, and he does the character such justice (the typically horny, female crazed 25 year old) that I forget that he is gay IRL. If he stunk at acting, it would be pretty obvious that he wasn't into chicks, and therefore the character would be totally unbelieveable.
For me it's got nothing to do with their sexual preference, and everything to do with how well they portray the character.
I happen to think George on Greys isn't as talented an actor as Barney, and you sometimes get the vibe that he's not as into the chicks as he could be, and thats where it falls down.
So in short, no, they don't have to be straight, they have to be good at pretending they're straight.