Unlike most other states that only apply marital law to legally married couples when they get divorced (including both gay and straight couples or those in marriage-equivalent state registrations), the courts in Washington State have adopted a unique approach. They are authorized to apply marital rules to unmarried couples, but they do so as the "equitable standard" for couples that break up, both gay and straight. Thus, the marital rules are not applied to every unmarried couple and they aren't applied rigidly, and so the judges have a great deal of discretion when it comes to applying the rules. The judges get to decide which couples meet the five factors for coming under the court's equitable marriage rules (cohabitation, duration, shared purposes, pooling of resources, and intent to be in a "marriage-like" relationship), and these rules can lead to some very interesting trials. Keep in mind that legally married couples don't need to meet any of these requirements, though most of them probably would if asked.
In a recent case (In Re Meretricious Relationship of Long and Fregau, 2010 WL 5071860) the appellate court upheld a ruling that concluded that the two men were in a relationship that met the five crucial standards. Thus, it was appropriate for the family court to have applied marital law to their break-up. Interestingly, one of the partners was still legally married to a woman during the first eight years of their relationship, but that didn't bother the judge. And, what is most interesting to me is that the court acknowledged that theirs was an open relationship, and then disregarded that fact. None of the judges had any problem in applying the equitable marriage rules to a gay relationship, and no one was bothered by the admissions of infidelity.
And so it appears that the concept of "marriage-like" has been modernized to include open relationships -- at least in Washington State. This result is fair -- especially since ever since no-fault divorce was instituted decades ago infidelity between married spouses is not supposed to result in any different distribution of assets and property. There is no reason gay couples should be treated any differently than straight couples with open relationships -- and at least in Washington State they are not.