Was out partying with a bunch for friends way back in 1980 something. After a few hours of drinking, my best friend was being a jerk to his girlfriend. Being quite drunk by this time, I was - how would one say this --- very relaxed. Anyways, his girlfriend came over to me and asked if I wanted to make love to her. It was so noisy and I was so "relaxed", I had to get her to repeat it 3 times before I understood what she was asking. I politely declined her offer.
A little later, I was dancing to a slow song with her and I explained that there were two reasons that I would not make love to her. First, she was my best friends girlfriend. And secondly (most importantly), I was so drunk I wouldn't be able to get it up anyways.
That was the first time I ever said no. It has happened a few more times over the years but seldom enough that it would be considered a rare occurence.