Somestime the spirit is willing, but the flesh is weak. I've been in situations where I've had things on my mind that effected my erections. One doesn't want to appear weak because, you know, maleness, and you just get anxious, which pushes the likelihood of success even farther away.
I was talking to @williek311 about it but I told a guy to leave once. He couldn't get an erection so I said oral or leave and he left bc he was embarrassed. I brought him home for sex not to cuddle.
A friend of mine is great with women but has ED. So he'll make a girl finish with his hands and mouth and the girls would be all like, "now the D?" He'll be like, "nope, just can't do it tonight," and take them home.