HUGE difference. He's actively having sex with a man. If he's straight he can ride a dildo. You are wrong, that simple He sucks cock, get his ass fucked, takes cum.
I've fucked a couple of straight guys. It's an amazing feeling. Not really because their ass feels different but because both were married, one was a dad and the fact that they wanted a guy to fuck them, then they go home to their family was such a turn on to me.