Person 1: hey, I was with Kayla last night and I blew it in her mouth. She's nasty.
Person 2: wait you left her suck your dick?
Person 1: yeah, I know, its risky with her braces but she didn't cut me up at all. I just put my seed behind bars.
In other words, if you hang out too long on da local pub-stools tossing down tall frothies, you will likely end up "cooling off" in jail after participating in one or more drunken brawls, staggering about in public, committing an OUI offense, etc.
Ethan Couch's infamous drunk-driving event didn't even happen as a result of his being in a tavern --- i.e., he boozed it up while partying at a friend's house --- but he still wound up in a similar "behind bars = behind bars" situation.
The act of ejaculating onto your partner’s teeth during sexual climax. However, your partner would have braces on, this coining the term “kids behind bars”