I'm so freaking sick and tired of everybody making that assumption. Almost every interview I've seen with Bryan, somebody brings up that possibility. It's not even possible for that to happen. Several times on screen it's been stated that Kate and Ian's affair happened while he was married to Madison. Either Madison's a cougar with really good skin (which would mean that her products are really as amazing as she claims) or people just aren't paying close enough attention.
I would be very pissed if the show actually goes that way. As far as I'm concerned, Lucas is a Horton and always will be. I hope they start recognizing him more as one. Austin, Billie, and Kate aren't the only family that he has. Hell, I'd rather be related to the Hortons anyways, lol.