Reinforce these discrepencies with plenty of cultural influences (has there ever been a TV show more frightening for dudes than Bridezillas?), and it becomes less cool for a guy to get tied down. The younger (or less mature) the guy, the more easily influenced – and therefore, cavalier about your relationship – he might be when around friends. In a nutshell, your boyfriend is likely masking his vulnerability, and the fact that he actually loves you lots.
Of course, he might also just have other interests with his friends that you two don’t share. If so, let him re-enact scenes from Jackass with his buddies; ignore his descriptive stories about flaming dog poop. But be sure to lay the smack down when you feel he’s out of line: don’t ever let him disrespect you in private or in public. And if he can’t learn from his mistakes, call one of your girlfriends up to make your own Jackass video by kicking him to the curb.