Okay, I'll concede that it's probably easier for this to occur if the 'lee is a woman, just on the basis of the plumbing. However, after a guy has cummed, who says the tickling has to stop? As a guy, he can't come more than a certain number of times in an hour, but when you gotta go, you gotta go. Eventually his kidneys will overcome whatever hormones are keeping him hard.
Again, biology is biology.
But in addition to that, I think the fun part is causing this sort of event to happen to someone who is generally dominant, someone who ISN'T used to being on the receiving end nor relinquishing control. Even for a dom, they may be really angry but they'll also be hornier than hell at the thought that someone managed to dominate them to that end.
And I don't find the scat part in the least bit erotic, but peeing is a different matter. If you wanna know why, there's a dissertation if you follow this link. It's quite educational without being vulgar or too formal:
http://www.patches.net/page/trainingman.htm It's called the Watersports Training Manual; however, the suggestions could obviously be enhanced with a tickling type situation. It goes into some of the facts about what urine is, what it is while it's within you, and what it is when it comes out, and then what is is once bacteria reaches it. This manual might also make you slightly horny while imagining tickling possibilities, so be prepared, lol.
