Despite attempts to evade these asymmetries with such fashionable phrases as "a pregnant couple" or references to "spouses" rather than husbands and wives, these asymmetries take many forms and have many repercussions, which laws attempt to deal with on the basis of experience, rather than theories or rhetoric.
Wives, for example, typically invest in the family by restricting their own workforce participation, if only long enough to take care of small children. Studies show such differences still persisting in this liberated age, and even among women and men with postgraduate degrees from Harvard and Yale.
In the absence of marriage laws, a husband could dump his wife at will and she could lose decades of investment in their relationship. Marriage laws seek to recoup some of that investment for her through alimony when divorce occurs.
Those who think of women and men in the abstract consider it right that ex-husbands should be as entitled to alimony as ex-wives. But what are these ex-husbands being compensated for?
And why should any of this experience apply to same-sex unions, where there are not the same inherent asymmetries nor the same tendency to produce children?