Re: The Power of the SCOTUS III: Roberts' Rules of Order
marriage started as a method for a man to ensure he was passing his property on to his own child and not some other man's.
A traditionalist's defense of the definition of marriage would include reclassifying a woman as chattel whose virginity is the property of her father before marriage and whose baby-making machinery is the property of her husband after marriage.
That's only part of the story. Marriage also imposes reciprocal obligations on the man to provide for spouse and child as well.
And you also note that heterosexual marriage started based on a biological imperative.
I guess everything old is indeed new again....we hashed this all out once before, civil unions between two consenting adults of whatever gender and sexual orientation are widely acceptable, and some people view the concept of "marriage" as something sacramental. Heterosexual civil unions are why we need marriage certificates and why many of us got married in a court house instead of a church (Catholics who divorce cannot get remarried in the Church while they can celebrate a civil union with a second spouse...I'm not up on my doctrine but if I understand correctly, a Catholic who divorced and remarried is fine under the law although technically by the Church's standards s/he is an adulterer/adulteress?)
and so most of the "debate" about marriage vs civil union is an issue of semantics, one concept is recognized by various religious doctrines while the other is recognized by civil law. As many have pointed out, heterosexual civil unions provide inheritance rights (even if one spouse wants to completely disinherit the other spouse, state law generally mandates that the "disinherited" spouse can still claim 1/3 of the other's estate, no matter what the will says); visitation rights, survivors' benefits, health insurance coverage, estate tax deduction at first death, etc. etc. Hence the drive for civil unions for same-sex couples, so that they also can have recognition of those same rights for their partnership.
I don't think "contract law" is an adequate explanation. For whatever reason, many civil societies, not just ours, decided that a biological nuclear family (or extended family, for that matter) provided generational stability and a healthy continuation of social life.
Other models are possible. If one believes the society described in Plato's writings, for example, it seems like all the men are bisexual, and the children are raised solely by the women, who were little more than domesticated servants who supported a male-dominated sociatey. The myth of the Amazons had it the other way around. All the women were bisexual, and the men were little more than domesticated servants.