Again, Rebecca is an "established" partner. In fact, she's the catalyst for Claire and Damon's sexual relationship. And there will be no problem with this because the audience to whom these works appeal is overwhelmingly male. Once again, you're trying to invalidate a stance by arguing hypocrisy when the female perspective doesn't matter. The audience likely wouldn't care much about Damon boinking Rebecca because it was already established that they had been boyfriend/girlfriend for five years, and that the resolution will ultimately lead to Claire being a "faithful" replacement."