It’s a trope: when two people have sex in a bed, the resulting bodily fluids will make the bedding wet, usually in a specific area. One partner will get to roll away from the wet area and sleep on the dry part of the bed. The other partner will have no choice but to sleep on the wet part of the bed (or leave the bed entirely, but that’s hardly romantic).
But… but how else can you tell if the pet sheets work?!