For me it was years ago when I was in the Army, at night while doing guard duty. My post was the battalion motor pool at the remote edge of the base. About 10 pm a car came down the perimeter road and parked. In a few minutes I heard a woman crying out "no, no..." so I shouted to the guy to get out of there. The car drove away, but left me thinking about the guy trying to get his cock into her mouth or cunt, arousing me so much I stepped between two trucks, opened my fly, pulled out my package and jacked a load there.