Isaac says he won't have sex with someone he's not emotionally attached to. He once walked away from a no-brainer—when a "gorgeous" former student "told me she sucks really good dick"—because he felt he couldn't be giving, as he was emerging from a bad break-up. Another time, turned off by a former lingerie model's excessively dental oral technique and her weird pillow talk, Isaac did the unthinkable: "I just pulled on my pants and made a run for it—out the door, down the steps, onto the street, and into the car."

When a guy hangs up on a booty call, he might be responding to simple biology. "As men age, the refractory period, or the interval between when they are physically ready for sex, gets longer, so they might not be as easily stimulated," says Jean Elson, a University of New Hampshire sociologist who studies voluntary abstinence. Harry Fisch, the author of The Male Biological Clock, says, "Testosterone starts to decrease at age 30"—so your sex drive decreases as your spare tire inflates.

Chemistry might also awaken feelings a man wants to avoid, further deterring him from casual encounters. "Sexual stimulation increases levels of the neurotransmitter dopamine, which is associated with feelings of romantic love," says Helen Fisher of Rutgers University's Center for Human Evolutionary Studies. "And orgasm causes a flood of oxytocin and vasopressin, hormones associated with deep attachment. So a man stands a chance of falling in love or getting attached when he doesn't want to." In other words, according to Fisher, "there's no such thing as casual sex."

But don't mistake a guy's unwillingness to score for an inability to seal the deal. "I never regret not going home with someone, but I frequently regret the opposite," says Brian Battjer, 30, a product manager in New York. "When you've had too much casual sex, it becomes enough to know that you could've taken a girl home—you don't have to actually end up going through with it."

Another New Yorker, "Greg," says he's forced himself to disengage from about 20 near hook-ups—sometimes naked, as when he suddenly found himself showering with a girl before deciding whether he was into her. (Nope.) "Sometimes I can't imagine whether I'll be turned on or off until we're in the moment," he says. "There's plenty of hot girls I've hooked up with who I should be turned on by, but in the moment I'm not." He once claimed cat allergies but got busted two weeks later at a mutual friend's house, where the Cat Lady caught him snuggling a tabby.

No matter what the excuse, a single woman spurned is likely to sound pretty miffed. "I had set my sights on this drummer boy who'd been giving me the eye," says "Amy," a 39-year-old comic. "He acted like we were going to do it, but when we were mid-make-out in his loft bed, he suddenly sprung on me that he'd only had penetrative sex with four women and wanted to keep it that way. I was pissed—because unlike most women, I really only get off from penetrative sex," she says. "What a stupid fucking night that was."