A large towel laid on top of bed sheets, designed for the sole purpose of having sex on, reducing the chance of soiling the bed sheets with bodily secretions.
A towel the size of a large hand towel that you keep handy to absorb "after sex" secretions. A towel that women use after sex to catch semen falling out of their vagina.
Man, after sex with Paul I always have semen dripping down my leg so I have to make sure I have a sex towel handy to clean up.