I don't believe I've even seen one, do they really exist or are they just a fable??
Hit the deck! It's gonna blow!!!!!
Probably because they really can't.
I'd start by telling her she is incredibly gross by sharing with you the fact that there are semen stains on her pillow. You defused the obvious weirdness by calling hair grease instead of the disgusting junk it was. If she's going to mess the sheets, it's up to her to figure out how to clean off the residue the next day. Ugh!!!! Tell her to do it outside on the lawn next time. It's easier to just hose it down afterward.
Get your cigarette lighter and a fire blanket ready for the big one. Mark and Spencer sell an excellent range of double gusset baffled, boil in the bag underpants for just this eventuality.