First of all, I think fairness is an issue. That the gov't selects one specific group to reward and refuses to award other couples or groups that provide the same benefits is unacceptable.
Numerous studies have shown advantages to having a stay-at-home parent for children. Having two wage earners and still having a stay-at-home parent is clearly an advantage. And those two wage earners are not supporting more with less. They have 2 wage earners for one family.
Whether the children are concieved naturally or through artificial means (or adopted) is not relevant and should not be a part of the decision as to who gets the gov't benefits. If anything, the gay couple would have to work to achieve children, whereas the straight couples have them by accident pretty often.
Exactly. A gay couple does have to go through several more hoops to achieve children, even to the extent that there are qualifications to be met for adoption that require proof of stability. The government need not add more encouragement to achieve that stability, unlike the more natural and often accidental addition to a more traditional household.
As for sanctioning of polygamy, I think that leads to more serious issues relative to the “benefits” of marriage. Would you require the first wife to agree before adding the second, or is it solely the husband’s choice? What happens to the estate built through the first marriage period that the first wife helped to create, is it shared three ways now? I think you’d find upon inspection that some of the 1400 +/- marriage “benefits” (I think at least some of the “benefits” are actually “responsibilities”

may or may not fit other situations as well as those for which they were intended, and many are no doubt in direct conflict.
The question of whether it serves public policy is debatable with very little hard data to support either side. However, I would offer that in a reasonably free society that that standard is inappropriate. If you wish to deny someone the right to live as they wish then the onus is upon you to demonstrate it is detrimental to society - not simply that it lacks benefit.
As to sanctioning, the government has determined it is appropriate for it to oversee marriage. In a legal sense, marriage is nothing more than a contract between competent adults. What is currently being done for both gays and polygamists, the government is intentionally discriminating against them by not allowing these contracts. That is changing for same sex marriage, but the discrimination still exists. I would go back to my first statement and argue that it is the discrimination which must be shown to be necessary, not the discriminated against. Unless there is a clear and valid reason to refuse to allow these contracts, then they should be allowed. I don't believe the fact that some folks find it "icky" constitutes a valid reason.
I don't think I implied "ickiness" was in any way part of the discussion, and I find that remark somewhat offensive. I don't wish to deny anyone any freedom to live as they choose. I'm not anti-gay; I haven't really considered polygamy as within the confines of normal society, so haven't really given that much thought either way. However, the government often encourages behavior if it is within the boundaries it has set. We encourage homeownership with a tax deduction and government sponsored mortgage enterprises, but only if you buy a home and live in it. The public policy goal is more stable neighborhoods. Do you find that unfair to renters or those who own multiple houses? Is it discrimination?
We encourage charitable giving, but only if your donation is given to a recognized charitable organization. The public policy goal is to provide for the welfare of citizens that the government would otherwise have to provide. Giving a $5 bill to the homeless person on the corner is not recognized and you receive no government benefit for that; do you find that unfair? Is it discrimination?
Marriage is also an institution with a public policy goal of stability and responsibility. I think if you want to change the terms, you should be able to state how that goal is enhanced by that change. After all, we’re not really talking about the “right” to marry, are we? I think a good contract lawyer could take care of most of the rights and responsibilities inherent in a marriage. It’s the benefits that are being sought.