Remus's excuse is flimsy, for such an intelligent man. Then anyone could have an affair just by rationalizing they were "teaching" someone and then blame their partner if the partner got jealous. I figure if Remus has to rationalize the sex with Teddy away, he knows he's doing something wrong, and it's not just the incest. He is betraying Sam's trust. And I find it interesting that I think betraying Sam's trust and cheating on Sam is worse than what he's doing with Teddy O.o
Oh, the other thing I wondered. I don't remember Remus talking to Teddy about diseases, or protection or nothing. I would have thought that would be the first lesson, even if Teddy may have learned it in school they would go over what Teddy remembered. Remus is so concerned about Teddy's safety etc in learning about sex from others, then why didn't they talk about safety?