Although i love that scene, both scenes like woah, i don't think they're possessive in the conventional. "he's mine, hand off" way. I think it's more a "you're with me tonight and i don't care who knows it" way.
it's part classic Kinney hedonism. Justin tastes/smells/feels good. Want Justin. and part "maybe i don't do boyfriends but i'm doing Justin, again, after spending the day and night with him."
I have never believed that Brian is sexually or romantically Jealous of who Justin fucks. I do believe that when he wants Justin, he Wants Justin, and anyone in his way can fuck off.