My friend Gary is prime dating material. He's smart, he's athletic, and he has a high-paying job in engineering. On a recent night Gary and I were at my neighborhood wine bar and I introduced him to a sexy, single female friend of mine. Everything was going great: They were laughing at each other's wisecracks and generally hitting it off. We got another round of Rioja. Then Gary (not his real name) got up to go to the bathroom.

"Gary's great, right?" I asked my friend.

"Yeah! So cute! Why aren't you dating him?"

"Oh, no, no, no. Gary's straight," I said.

"Uh, no, he isn't," she said.

"Yes, he is."

"No, he isn't!"

After that night, no matter how hard I tried to convince her that Gary was as straight as an Arizona highway, she refused to believe it. It wasn't the first time this had happened. In fact, almost every time I introduce Gary to a woman—or a man, for that matter—I'm asked if he's gay. "I wore these new Alain Mikli glasses to work, and it was as if I had worn a dress," Gary told me once, explaining the effect he has on his peers. "All the guys kept saying with lisps, 'Oooh, look at your fanthy glatheth!'"

Gary's what I like to call a "stray," a straight guy who sends out gay signals like he's shaking a tambourine even as he proclaims himself—and in fact is—100 percent heterosexual. The characteristics that define a stray as such vary widely. Maybe it's a melodious laugh. Or a fastidious shirt-and-tie combo. Or an effusive signature salutation ("Oh my god! I'm totally psyched to see you!"). But more often the thing about a heterosexual guy that makes everyone assume he's a homo is almost impossible to pinpoint. He may talk up his love of ladies more than Bret Michaels does, he may have a wife and kids, but people always react the same way: "Really? No, wait—really?"

This phenomenon shouldn't be confused with that of actual gay men who masquerade as straight. And we're not talking about the metrosexual, that embodiment of a played-out consumer megatrend that involved slim-cut pants and moisturizer. Every guy in America knows how to clip his nose hairs and make his outfit go from day to night. Those skills aren't what make a heterosexual man read gay. So what does?

Science has tried to figure this out. Researchers have studied behavioral traits like "hip sway" and "voice quality" and even physical traits like hair-whorl patterns and finger-length ratios. Richard Lippa, a professor of psychology at California State University, Fullerton, and the author of Gender, Nature, and Nurture, says that no one trait can be used to determine someone's sexuality. But, he says, "I think the one thing we can conclude from the existing research is that both scientists and laypeople can judge people's sexual orientation at better-than-chance levels based on behavioral traits."