It was certainly not because you didn't turn him on. It has nothing to do with you. Gay? Why would you suggest this? This is mean of you to even mention it.
It is not uncommon for this to happen to guys and those that say it's never happened it will at some point. If this is his first time then you can blame it on nervousness. If you think there is a lot of pressure on gals it is nothing compared to a guy's first time. After all, our egos and our penis are hot wired together.
Sometimes there are other things that cause impotency to happen, such as; drugs and alcohol.
My suggestion to you would be to forget about it and pretend it never happened. Trust me, he is humiliated enough.