Ok, very brief rebuttals, since you insist.
On adoption: I think adoption is great. I even think single parents adopting is great (if the alternative is no family).. I think arguably adoption is morally better than procreation. (The Christian view is we are all children of God through adoption).
But before you can adopt you have to have parents who have abandoned or are judged by the state to be incapable of caring for their own children. Adoption is thus a happy end to a tragedy or a crime. I personally think there should be preferences for married couples in adoption law. I could be persuaded you ought to have second-parent adoption laws for gay couples. But I don’t see how the fact that some married people adopt undercuts the relationship between marriage and procreation.
Anymore than I think the fact that people can and do bear children outside of marriage (and the state doesn’t forcibly repatriate their children) means that marriage really never really been about procreation.
People raise the most bizarre arguments: Well, if marriage were really important for procreation and family structure we’d do “X.” And the “x” is something no marriage culture has ever done to my knowledge: like forcibly annulling childless couples.
Andrew Sullivan is particularly good at this: He pulls out of his hat his standard for what would make marriage connected to procreation, like testing couples for infertility and barring those who are found not to be procreative, or having the state end marriages that are not child-producing within five years. And then he declares that the fact we don't do this "proves" marriage isn't really connected to procreation in law.
But the way marriage cultures work is quite different: by separating out a certain kind of sexual unions—husbands and wives-- and surrounding these unions with special legal, familial, religious and cultural support. Because the way it works in reality is, the more people attracted to the opposite sex who enter such unions, the better off children will be.
A subtler argument sometimes made is this: well, we have some nonprocreating couples in the mix. Why would adding SS couples change anything? Two points: SS couples are being added to the mix precisely in order to assure that society views them as “no different” than other couples. This intrinsically means (if the effort is successful) downgrading if not eliminating the social significance of generativity (procreation and family structure). The second truth is that both older couples and childless couples are part of the natural life-cycle of marriage. Their presence in the mix doesn’t signal anything in particular at all.