a lot of times i think up funny scenarios in my head like what if cia agents were to bust in the door drag him away and he'd have to be interogatted with a boner.....
Once in high school when my gf and I were making out in a park at night, suddenly we were raided by a party of policemen with flashlights. They said they were "looking for runaways," but I think what they were really doing was looking for teenaged girls who were partly undressed. Oh, ummm...the point being that I got to be interviewed w/ boner by law enforcement. But no, I did not laugh. Nor did my gf.
I have before. Generally if I have it's because I have a good relationship with the person and we feel comfortable with being goofy and making each other laugh. My bf now gets really really sensitive after sex, so sometimes even while it's still going on, if I touch him a certain way it sends him into a fit of laughter. ^_^