Expert opinion may clarify facts, but says not thing about cooperation. Gay marriage is not a matter of fact but a matter of contract. The government forced an involuntary contract (again).
My opinion is that 1) constructively, marriage consists of (a) reciprocal power of attorney (b) formation of a corporation into which both parties contribute the totality of their assets. (c) reciprocal insurance against vicissitudes of life.
And that (2) the origin of marriage appears to have been (a) retention of property as the division of labor (merit) increased. (b) a means of preventing fratricide, (c) a means of controlling female reproductivity by preventing her from forcing costs onto the tribe, and (d) an institutional means of absorbing lower class males.
And that (3) marriage remains either (a) a corporate long-term investment, (b) utility for those who have property, and (c) a disutility under redistributive government.
And that (4) the purpose of gay marriage can be solved by (1). And that the difference between gay and heterosexual marriage is substantial for the reasons listed above.