I'm afraid to translate this one literally, I'll get reported for questionable content.
So the clean version is: sex-in-town.
It actually refers to a small bag, like an overnight bag or even smaller, in which one might carry all the necessary items one might need were one to decide to not go home one evening in favor of spending the night elsewhere pursuing pleasure.
How's that for clean? Very, I'd say.
It's the kind of word I love, not necessarily for what it represents, but simply for the fact that there is a word for such a thing. I love that about French.
I guess the next obvious question is what you would put into your baise-en-ville...