Multiple relationships are acceptable socially in many segments of society. The only problem being that the law does not recognise them. Until my wifes death, I was part of such a relationship. My lady Meghan was married to me and also had a committed relationship with her lover Genevive. It was essentially a variation of polygyny, a women having two partners, one of them being same-sex. Each of our relationships was with Meg, Gen and I were close friends but did not consider ourselves bound by a marital-type relationship. Genevive was mostly oriented toward women and Meghan was bisexual. The three of us frequently slept together, but I always repected Genevives orientation and her limits on the degree of contact she could tolerate with a male.
At one point we had considered having a child. This would have been a complex process as Megs tubes were blocked and abdominal injuries would have made pregnancy dangerous for her. This would have required extracting ova from Meghan, reversing a vasectomy or extracting sperm in another way from myself, IVF and implanting the fetus in Genevive. If successful, this process would have resulted in a child with a daddy and two mommies, a more stable situation than a “traditional” family.
I strongly believe that marriage should be defined by those entering into it. The only legitimate function of government being to record and archive the agreement and mandate that any children resulting from the marriage be properly supported and cared for.
Multiple relationships can be more economically stable and provide for better child care; two wage earners and a stay-at-home parent for example. Other forms of multiple relationships, such as “line” marriages would result in a family becoming increasingly more financially stable across generations. See R.A.Heinleins “the Moon is a Harsh Mistress” for examples of alternative marriage arrangements.
Another benefit of a multiple relationship is that if a disagreement occurred between two of the partners, the third could act as an arbiter. If course, with any such relationship I’m describing, all partners would have equal rights, unlike some forms of polygamy where the male holds greater power. In our relationship, we all had equal rights but specialized responsibilities; I was the primary financial provider, Genevive ran the farm and Meghan was consider head of the household.