"Would you have sex with your best friend?"
"Ew, no. He is like brother. That's incest."
"Would you have sex with your best friend's boyfriend?"
"That is so wrong. I'll kick his balls if he dares to ask."
"Would you have sex with Keanu Reeve?"
"I can consider him, but he is a bit old. But Shayne Ward, now he is hot."
"If your best friend or his boyfriend is Shayne Ward, would you have sex with him?"
The only reason one gay man do not have sex with another man is because he is not sexually appealing. It's not because he is straight, or because he is a close friend, or that he is attached. The plain fact is that he does not stir your groin. There might be a slight difference between thinking about it and actually doing it, but that difference only matters if you are attached. Even so, why think so highly of yourself?