I'm definitely not into underage stuff, Billy/Mrs. Wheeler included, but this is very, very mild as far as underage goes. He's seventeen, and he appears to be quite sexually experienced. In fact, I bet he has way more sexual experience than Mike's mom, who married her highschool sweetheart. Not that him being almost legal and quite sexually experienced makes it okay for her to have sex with him. It's still wrong. But unless the show tries to pass it off as a good thing that she's having an affair with an underage boy, I have zero problem with the storyline.