It seems like this is a question that should have been asked in the thread "Questions That You Were Afraid To Ask".
But to answer the question, I did once when I was 16 years old, and back then all it took was a gentle breeze to get my dick hard!
I had just gotten my drivers license and went parking for the first time. My date and I were making out pretty hot and heavy engaging in the "Roman Hands and Russian Fingers" activity thing, and I had a boner that was just dying to get out of my pants! But while she was rubbing my dick I guess the excitement and anticipation was too much for me because lo and behold I blew a big messy load right in my pants!
In case if you are wondering ...After a 10 minute break and a quick smoke we went at it again, and this time my dick made it into her awesome red hairy pussy, which was as wet as the load I shot in my pants!