Sometimes from other "straight" men - such as long distance truckers. This obsession with what is gay and what is not is part and parcel of how you have to prove you are a man in this culture by demonstrating that you are not in any way like a woman. And, as others have noted in the comments, only a fraction of gay men even engage in anal sex. It's all just utterly silly, if you ask me - which is why this is a humor piece.