Mannerisms are a funny thing. They vary greatly from one part of the world to another, and even from one part of the USA to another. It depends on the local persona. What's butch in one place may be considered fruity in another.
When I moved to California from Texas, I thought about 80% of the guys I met were gay. The way they spoke and acted (I mean "behaved", semantics Nazis), if they were in Texas, there would have been no question that they were gay. But in truth, they were virtually all straight. And in California, they were all considered to be "straight-acting". Obviously "straight-acting" is defined differently in California and Texas.
To me, "gay-acting" means you're giving clues to your sexuality in your speech or body language. It's not necessarily effeminate, and it's not necessarily a conscious effort - most guys, no matter if they're gay or straight, simply adopt the mannerisms of the people they hang around with. But a subtle inflection in speech or a posture of the body can telegraph sexuality. "Straight-acting" - or maybe we should more genuinely call it "straight-appearing" - is simply the absence of those clues. And it's no big surprise that a large number of gays prefer "straight-appearing" men - considering that's how all the male role models we've been subjected to our entire lives have acted. Guys like Richard Simmons don't get cast to play Indiana Jones.