Dear OBB readers: My fiance has had gay sex, does it mean he's gay?

From a femaleOBB reader

I am engaged to this guy who confessed to me just last week that he has had gay sex in the past. He actually confessed after someone else told me about it. He said he did it for money and not because he's gay. He said his former boss was into men and lured him to have sex with him and after a few romps, gave up on gay sex. He says he's 100% heterosexual but I can't get over the fact that he once let a man sleep with him. He said he was tempted by money but I don't believe any straight man would have gay sex for any reason. I don't know how to feel about this. These days whenever I see him talking to a guy, I start to imagine them having sex. A man who has had gay sex is gay, right?