One of the things that I think was missed in the same-sex marriage 'debate' (there was no debate, just invective from the pro side) was Gen 2:24. The two become one flesh.
I recal some talk of the complementarity of the sexes, and that the relationship of marriage resulting from the dynamic between the sexes, between man and woman (who together are in the image of God...that wasn't ever in the media, of course).
The complimentarity runs deep: men and women are just different, different in ways that make each important to the other, and that 'interlock'. The physical union expresses and demonstrates this.
By comparison two women or two men do not 'interlock', they stand side by side. The 'interlock' metaphor runs through the BCP/AAPB marriage service and is demonstrated in society: one's relationship with one's spouse is more important than any other. The two share their lives as one: thus sexual loyalty is imperative. This is the building block of civilisation (or the tribe, in generations past).
There are deeper things here too both psychologically and socialogically for the couple, their children, their wider families and communities. It would take a good length book to explore them! The spiritual dimension would be another book.
None of this surfaced in the debate...because there was no debate, mere ranting from the side-by-siders.