Barry is annoyed that Serena convinced him to let his daughter date a man whore whom Dan claims -- correctly -- has dated not one but TWO con artists. He's also understandably wigged out to learn here, for the first time, that his girlfriend and his child have both had sex with the same person. Serena is like, "Girl, please, are you new?" So that Barry can get to know Nate, they all decide they'll go to a horse show together, because that's how these people bond, and Serena is allowed to get back to wondering why she's wearing a dress that's so unflattering to her hips.