I don't think you can really make an argument based on laws. Those arguments end up with citations of scripture, with one side interpreting it one way and the other a different way.
This really seems to be an area where philosophy becomes important to theology. We have to ask, what is the purpose of marriage- what was God establishing in humanity and what does the bible reveal about the nature of sex.
Western culture seems to hold the position that the purpose of marriage is to satisfy an innate desire for intimacy with another person, expressed through sex. However, this contra[di]cts what we can reason about the purpose of sex and marriage within God's revelation. It's not a mutual agreement between two people seeking intimacy- it's a calling that directs us to a higher part of purpose- procreation.
Homosexual marriage corrupts the purpose and meaning to marriage. Sins are, after all, when we do something that goes against God's will and plan for us. It's generally not the complete opposite, but a deviation from it. Gay marriage, pre-marital sex and other sexual sins are problematic in that the seemingly fulfill part of God's plan, but ultimately take us off course.
You make an excellent argument here, one I wish had been dealt with much earlier in the thread. I am going to concede a great deal of what you say in the earlier paragraphs, and then bring up the concept Catholic theologians have advanced regarding marriage. That is, there are
two purposes in marriage: the procreative and the unitive. It is, on the one hand, for the conception and nurture of children, and on the other, the fulfillment of God's plan in giving us the desire for intimacy in the first place. For the Catholic, these must run together -- neither alone is sufficient. If marriage were
only for procreation, then the infertile, women after menopause, etc., would be excluded from it. If it were for intimacy only, then the whole element of the proper nurture of children would be left unfulfilled.
However, I submit that this is
normative rather than
mandative -- that God does create people for whom normal one man/one woman marriage is improper. (The verse A4T mentioned earlier about eunuchs, generally understood to mean there are those who espouse celibacy as His call for them in particular, supports this.) My wife and I were not blessed with children of our bodies -- but He was able to lead us to understand that our call is to minister to others, together, including children alienated from their natural parents, and to comfort, counsel, and nurture those people, including those children, using the gift He gave us of our mutual love and support to sustain that ministry.
Marriage is, in my mind, for the creation of families. But that is, in my view, not just limited to two-parent-plus-kids families -- all those whom He has called together are included. The man who adopts children as a single parent because he has parental love to give and they are in need of it, and finds fulfillment in nurturing them. The gay couple who adopts children and raises them. The elderly couple who takes in their own abandoned grandchildren and resumes the parental role in their later years. These are all real and healthy families, and ones that don't meet the very limited "family values" definition.
Sin is failure to love God with all that is in us, and to love our neighbors as ourselves, and to show that love in very practical ways -- where there is hurt, we are to comfort. Where there is sadness, we are to bring joy. Where there is injury, we are to heal. Where there is strife, we are to bring peace. Running down a shopping list of "Thou shalt nots" is not cataloguing sin; looking at the hurt in the world is. And when we have done that, our call is to relieve that hurt. Anything less is arrant sin.