"Una and Geoffrey are having a Tarts and Vicars party"
Tart and vicar
Creative Commons Attribution Share AlikeTart and vicar - Credit: dionc

A tart is a prostitute or a woman with a carefree attitude to sex. A vicar is an Anglican parish priest. 

Tarts and Vicars party involves dressing up as either, regardless of gender, which perhaps goes some way to explaining the continuing popularity of this well-worn theme.