It's the sex that you have when you're in between relationships. It bridges the gap(or lack of sex in a relationship) after a break up and before you find someone new to have sex with in a relationship.
Rodney - Damn, since I broke up with Holly I haven't had sex at all.. I really miss it!
Nicole - Oh, you poor boy! Why don't you come over later and we can have some Bridge Sex! That will make you feel better, and me too!