My perception of Carrey's portrayal was simply a loose set of known "gay characteristics" - I don't think it was realistic, otherwise.
McGregor's portrayal was, I think, closer to a more standard straight actor's portrayal of a gay man. At times it worked, at times it didn't.
As I've tried to explain, my objections were not even fatal to my enjoyment of the film. It's more of an observation that homosexuality is now seen as a fit subject for parody, where a few years ago, race would have been treated similarly.
And when I said above that there were no easy answers, what I meant was that I have no objection to films being based around the lives of gay characters. However, I think a more mature view would be that the character is a person in their own right, and their sexuality is a facet of that.
I'd cite Colin Firth's performance in A Single Man as an excellent example of someone (known for very heterosexual performances) getting completely inside a role, and bringing it to life in a genuine and moving way.