Ian Gallagher has never really had any jealousy problems. Hell, Kash was married and had kids and Ian never had a problem with that. So when he and Mickey Milkovich started hooking up he thought Mickey wouldn't care either, did seem like the type who would, but when Mickey caught Ian and some random dude hooking up he was pissed.

“Mickey, what the fuck!” Ian yelled. The guy Mickey practically jumped running off. Ian was pissed. Why the fuck did Mickey even care? Bastard.