This would lead to a variable amount of cum. Since guys (I assume in this scenario?) would cum as much as they peed, then their volume of cum would depend on how much liquid was waiting in their excretory system. This leads to a wider range of cum volumes: an example being if some guy got done with a long gaming session in which he drank a lot of soda and really had to go, he would cum excessively more than if he had already gone before going to bed with his lover.
That said, I'd bring those 36-packs of bottled water to bed with me every time i had a guy over, and have my washing machine ready for the morning.