Why not just cast a homosexual lead?

So, Americans play Brits, Brits play Americans; Women have played men and vica versa; White men have played Black men and Black men have played white chicks. Gay men have played straight men and vica versa. I'm not ostensibly saying that it is wrong that Mark Ruffalo (being a straight man), being cast as the lead in this is wrong.

I'm not bashing Mark Ruffalo here; even though I think there were times where he played the part well, yet there where times he played the part totally cliche'd and over-exuberant. Whilst watching this film, I couldn't help but think 'This is Mark Ruffalo, trying to act gay'. I couldn't take him seriously. Perhaps I'm not watching this film as I should be, perhaps I'm being too critical...

Now, to put a bit of texture as to the angle that I'm coming in here. Whilst apples and oranges, are in fact, apples and oranges. When I see a film, and I see, for instance, a person acting the part of either a demographic, an archetype, heritage or cultural background that I am a member of. I look for how they represent me as a subset. Example; I am a Scouser, from a region in the north-west of England called Liverpool. I have Irish heritage, I am from a working class background and I would identify myself as a non-religious, left-of-centre, Dyslexic male. On occasion, I have seen people from all walks of life, portray one of any number of associations that I align myself with. Some (in my opinion), good, and some bad. What defines as good and bad is obviously subjective; however, when I see someone portray in a way where I can see that they are attempting to 'act' in a way that the vast majority will recognize. I wonder if there was/is someone within that subset that I associate myself with, who could represent the part in a way that isn't artifice.

I understand the concept of Drama and Acting, so it's not as I am ignorant to the art-form. I have a HNC in Performing Arts, a Degree in Drama and a Masters in Fiction Writing. So I'm not being a bore here. I understand that Home Box Office will want to get someone to do the job who will perhaps do the best job that they can. I understand that HBO want to get someone with gravitas and standing who will take on the part. I know that the writer or producer (Or Director and creative lead), may want this piece of be acted in a certain way.

So with all this, I still ask myself, why not just cast a homosexual lead?

Okay, so Tom Hanks is not gay and he played a gay part in Philadelphia. Half of the cast of Angels in America aren't gay and they played gay parts (Some are gay of course); Kevin Spacey is gay and has played many straight parts. And then there is Ian McKellan; he is gay and has played both gay and straight parts. Nathan Lane is gay and has played a straight man in a 'gay related' play. Sylar is gay and... Okay you get the picture, Hollywood goes round and around, the balance is more or less restored and the curtain is still called at the end of the day.

But in a piece of film that is so important as this, reflecting a real-life tragedy, with real people with real issues. Would it be so hard to just cast an openly gay lead?

I know the basis of argumentation and I know that differing points of view may be posited and posted to further or retort this viewpoint. I know that I may be looking at this through rose-tinted glasses and a certain degree of naivety. I know that there may be some people who may just see this as being argumentative for the sake of it. But I have a genuine *beep* question, from a genuine place, with a genuine curiosity.

So I ask again, why not just cast a homosexual lead? - What is it with people trying to get people to 'act a part' for the sake of 'artistry'?

"Oh that Mark Ruffalo, he'd be good".