why don't Brett and Jake make a life together without sex?

Jake asks Brett if they could just live together (since they cannot have sex, they could not be a real couple), but Brett says no, she would just cheat on him. Jake says she cheats on him now, and he's okay. But he really isn't, and she knows it. He asks if they could go to the country, but she says she wouldn't be happy there. Loving each other is not enough. Brett changes the subject, telling Jake she's going away for awhile, to San Sebastian, and then Michael will be back with her. Jake, hopelessly, asks again if they could go away together.