Practitioners of open relationships don't consider them adulterous  because the external activity is consensual to the non-participating spouse. They don't consider it cheating. The whole concept of adultery was forged when wives were the property of their husbands.  No longer the case.  Indeed, what scares the Church about both gay marriage and modern marriage is that the man is no longer in charge, which messes up their marital analogies about Jesus and the Church and the Hierarchy standing in Jesus stead as head of the wife.