I was at the climbing gym last night and a guy was doing these pull-ups on a hangboard with lock-off; pull up halfway and hold for 3 seconds, all the way, hold 3 seconds, halfway down, 3 seconds, all the way down, 3 seconds. Harder than normal pull-ups, but great for climbing since lock-off strength is an asset when you're climbing.
Anyway, I made some comment about how it reminded me of the "300" workout those guys did, training for the movie, or whatever, and another guy said, "I haven't seen that yet."
"Man," I said, "those guys got in SHAPE. They were unbelievably hot."
"Well," he said, jokingly affecting a lisp, "I guess I better go see it, then."
That was vaguely uncomfortable, but I just quipped back, "Hey, don't knock it. They've got plenty of naked girls for you guys who prefer the ladies. Equal-opportunity!"
He did the little registration-pause -- I've seen this a lot, straight guys very often do it when I mention being gay, bring up a boyfriend in casual conversation, etc. Usually I bring it up in response to a direct question or something like that, and they always kind of laugh a little, but then pause for a split-second, registering what I said.
The key there is that I treat it all as a total non-issue. I don't act awkward or uncomfortable, I just keep talking, and their little flinch-pause passes and they're fine.
Like Chuck says, making it clear we don't bite is key. I'd add to that: going out of your way to behave in a comfortable manner about it yourself will put them at ease, more, too. If you blush bright red and get incredibly awkward, they're not going to be the ones to bring it back down to a casual level, but if you really give off the air that it's no big deal, I find most guys are really quite fine with it.
I tend to use the same rules Chuck does. I will almost never spontaneously bring up my sexuality -- but, really, straight people don't, either. They might say a girl is hot, but they won't just out of the blue say, "You know, I'm really very heterosexual." Similarly, I'll make small comments about how ripped a guy in 300 was, or whatever, but not be like "Man, I would love to SUCK HIM OFF RIGHT NOW." Talk about making everyone uncomfortable.
But if anyone asks a question that kind of begs the response, then like last night I'll just tell them, and make it no big deal. A coworker asked about my last boyfriend, because I mentioned living with him at lunch when talking to a friend. "Oh, do you have a roommate to help pay the mortgage?" I laughed, "No, I just live with my boyfriend. And he doesn't help pay the mortgage anyway."
Immediately he did the half-laugh, registration-pause, and then lunch continued as normal.