Andrew Goddard's article is about trends and changes. Marriage has become more personal and part of a choice of options, including co-habiting with children reared outside and inside. Civil Partnerships have created parents-in-law, uncles and aunts so have added to changes.
Quite how this supports the institution of marriage I do not know. It seems to be an argument about the decline of marriage and its reduced relevance. His argument for seems to be to recognise male and female as complimentary.
Recognition of differences as well as similarities is vital – we would not wish all motor vehicles to be identified as "cars" or all colours to be classed as "blue".
Yes but marriage is a vehicle and offers some colour to life.
Compare the rather weak argument so offered with that of Jeffrey John. For him, manogamy is the key and marriage is becoming the shorthand name for both heterosexual and homosexual partnerings and calling it the same gives equal respect across both. Then he is straight in regarding the partnering as a sacrament, the covenant of love between Christ and his Church and a channel of the love of God. Nothing here about trends and changes. Plenty about the follow-on about social stability, including from gay people themselves, but principally it is theological:
He refers to the Bible, holding no concept of sexual orientation, and Paul seeing gay activity as heterosexuals working out their excess of lust. And Jeffrey John comes to this view as an Anglo-Catholic, not as a liberal. In terms of the Church and its reputation, its sidelining of gay partnerships is a disaster, distancing the Church.
"God saw that it was not good for man to be alone."
One wonders why Rowan Williams can't produce such an argument, instead of kowtowing to evangelical selectivity that there is no pro-gay stance in the Bible and some Mind of the Communion way back in 1998, that was never one mind at all (and indeed he didn't share it).
My own take on all this is that your sexual relationship should be one of your religious relationships, and that involves making a commitment of openness to the other. I made a commitment (unfortunately she walked off or never returned) and that was also a public statement and it was one to one. I wrote the service and went to the bother of having it in the Unitarian church to emphasise its holism regarding all else in my life including as a social statement. However, I personally have no view against people who have multiple partners so long as they are all affirming and non-exploitative. In reality, many do have one form or another of polyandrous relationship. There are married people with loving friendships elsewhere, and people who express themselves sexually among more than one person, sometimes at the same time and sometimes with more than one. Relationships are very complex. What undermines them is having more than one partner when the other expects you to be devoted to one only, and not hiding the truth. There is nothing as dishonest as the family tree, which must be a very cleaned up public record.