One morning a couple of years ago, Tyler Barnett woke up with a woman he barely knew. No surprise there: He was just your garden-variety twentysomething guy, a P.R. man in Los Angeles who enjoyed casual, commitment-free hook-ups. Yet sometime before dawn, something in him shifted. And once the sun was up, he decided he was over casual sex. He chased her out with a flimsy excuse: that he had to get to work.

Sure, Barnett was concerned about STDs and pregnancy. But mostly he'd gotten fed up with putting out. And he's not alone. These days, in seeming defiance of male sexual stereotypes, the guy is suddenly the one saying no.

Is a man who rejects casual sex a freak of nature? Increasingly, he's not. Putting aside any situation-specific reasons—she's too drunk, she's a stalker, she's got a goiter—some guys are finding they don't like how casual sex makes them feel.

Barnett, for one, has no qualms about screening a booty call. "I got a text this weekend from a girl: 'What are you doing tonight, I miss you, I want to come over.' We've had sex a couple times, but I didn't want to go through the whole song and dance, having to worry about her sleeping over. So I didn't respond."

Ben, a 30-year-old account manager in Cleveland, says that in his twenties he used to screw "like it was shaking someone's hand," but now, he says, he's had it with sleeping around. "To do that consistently over a long period of time," he says, "you really have to be emotionless, like a robot." Ben came to loathe the ambiguity of casual sex. "After going out a few times with this one woman, she let me know we'd be better off as friends—but two days later, we're having sex for the first time," he grouses. "It's like, Where are we now?"

Feeling vulnerable, that's where. "It's hard to have casual sex without getting emotionally involved," says Ben, who occasionally goes so far in his just-say-no approach as to abandon a girl at a crowded bar when he senses an impending hook-up. "Eventually, one of you is going to get involved. And in my experience, it seems to be me."

Vulnerability is hardly the only reason a guy might take a rain check. Maybe he doesn't want a gift that keeps on giving. "The number of women who will just sit on a condom-free, erect penis without any kind of announcement or discussion is just shocking," says "Isaac," a 33-year-old L.A. artist who recently finished a one-year sabbatical from casual sex. (Some names in this story have been changed.) "They would just go for it: no latex, no discussion of STDs, not even 'It's okay—I'm on the pill.' Dude!"