Joe asked me to make a post about a most unlikely question. "Do you think all effeminate men are gay?" Oh boy. Well, i'll give it my best shot.
Short answer: no. It's a free country and you can play with gender roles how you like. A lot of my straight-male friends are very effeminate, a là 'ohmigod that dog is soooo cute'. Even more of them act effeminately to make jokes with me and others.

Don't get me wrong - the thought has crossed my mind many times when i meet a guy who is impeccably groomed and speaks with an unmistakeably gay lisp, or talks about his male friends with female pronouns. What's up with that?
My theory is that effeminate behavior is an old left-over in the gay subculture's freezer. Before the coming out movement, there was a way to tell someone you were, you know, *different* without actually saying it. A flick of the wrist, an affected vowel here and there, you get the idea. At least i'm pretty sure that's how it happened in the cities. Feedback would be welcome.
And of course there were the bars, where men and women seeking a different sort of excitement found eachother. Before the solidarity of gay pride realized in the Stonewall Riots, a subculture had already formed in these bars. Part of being in the club was acting like the other members. All the cool gays and lesbians were breaking gender norms, so the rest followed suit. After all, imitation is the sincerest form of flattery.

Then again, it could just be a giant extended middle finger to a society that rejects them. It was and is an unwritten rule that only and all men shall act masculine and pursue females, likewise only and all women shall act feminine and pursue males. What better way to poo-poo some silly rule than to mock it?
And in true post-modern fashion effeminate straight guys and masculine gay guys are pretty much accepted. It's a much more ambiguous world, but more mature in my opinion. We're so serious all day using gender roles properly... but we can play with them, too.