To act in a homosexual manner when one's (supposed) sexuality is heterosexual. Not quite the same meaning as to "turn gay", but almost.
Dude A: "Dude, what's the deal? I wake up and find you sleeping in my bed! Why don't you sleep in your own? I hope you're not startin' to fag-out on me!"
Dude B: "Don't worry about it, man. It'll only happen once-in-awhile —usually after I suck your cock, like I did last night. And thanks for being cool enough to pretend to be asleep!"