Laughing in the middle of a session when something goes wrong can also save egos. Some people might think it's the opposite - 'they're laughing at me' style - but generally, bursting into fits of giggles because something goes wrong is so much more fun than swearing and getting all het up about it. You can just pick up (or pick the other person/implement/rope up) and carry on. Or say 'the hell with this, let's try something else' and not mind.

As far as humour in writing goes (Clevernick) - that's an interesting comment. I think also it depends on whether you decide that the genre you're going to write is humourous erotica/porn, or serious. I guess if it's serious, you want to capture the serious readers' imaginations before you make a joke. Otherwise they might not bother starting.