Maybe if he'd grown up someplace normal Seth would feel guilty about fucking the guy who basically amounts to his adopted brother, but he grew up in the O.C., and all it means is that he's finally joined them. He always swore he wouldn't, and suddenly here he is – he's crossed over.
If his dad hadn't joined them first, maybe this wouldn't have happened. But the moment his dad sold out, Seth knew it was the beginning of something. Two weeks later he walked in on his dad fucking Jimmy Cooper.
This is what happens in the O.C. Everyone knows about it but no one says anything. This is how this place works. Put on a plastic smile and cling to your wife or husband's arm and play happy families. And behind the scenes, everyone's fucking everyone else.
His own family is so tied up with the Coopers that he'd need a chart to keep track of who's slept with who. He considers making one, sometimes. Julie Cooper has not only had his grandfather but his aunt, too – while she was working as a stripper, too. He's probably not supposed to know that. There's a lot he's not supposed to know.
He can rant about the hypocrisy all he likes, and maybe he would have done, once upon a time, but the second he kissed Ryan he'd joined them and he knows it. No one knows that the boy who lives in the pool house is sleeping with the boy who was born into this family, because as far as everyone else is concerned, Ryan has Marissa and Seth has Summer and that's all there is to it.
And in public they smile and pretend, and follow the adults' example.