sloppy seconds
Alas, this word has now leaked into general usage and people are taking it to mean 'ill-fitting hand-me-downs'.

Get the truth out. It does actually having sex with a woman who has just had (obviously unproteced) sex with another man for the simple joy of sliding about in his juices mixed with hers.
Hey, you shag 'er first man, I fancy a bit of sloppy seconds tonight.