It depends what you think marriage is. I got married in a civil ceremony, after always expecting to marry in a religious ceremony where the contract is in the presence of God as much as in the presence of the community. To my astonishment I realised that this verbal contract I was making with my future dh was as meaningful to me, and as important and as binding even though God wasn't involved.
Frankly, same-sex 'marriages' are not acceptable in main-stream religion, but why should they not be acceptable to the community? If they are an expression of commitment and intent to tie one's life and one's future to that of another person, then why should some people be barred from doing that? I don't agree that 'co-habiting' partners shoudl have the same rights as married partners. If they want those rights then they should get married. Equally, if same-sex partners want those rights, then why shouldn't they have the option of getting married?
And I agree, I too have seen discussed research that says that in general the children of married parents do better than those of unmarried parents. Of course there are many committed unmarried cohabiting parents with stable and successful children, as there are the opposit too. Research just shows the overall picture, not the specifics.