This is so embarrassing to post but.... omegaverse, in which the two characters fuck and knot, and post coitus, one of them has to piss. however, the firm hold of the knot makes then a) have to hold it until the knot dies, and become absolutely desperate or b) they end up accidentally pissing themselves on or inside their mate. The desperate one can be either the omega or the alpha, it doesn't quite matter.

As far as ships, OP has a preference for USUK, but but most ships are fine. Preferably no RusAme.