From a song from Jonathan Coulton, talking about a party. A town crotch is a person thats "good to go at anytime". Ready to have sex at anytime.
See also: town bicycle
(sees a slutty chick) Guy1: "Heey bob, there goes the town crotch!" Bob: "Hey now, thats my sister!"
girl1: "this is like the 6th girl he has went out with this week."
girl2: "sounds like we have a new town crotch"