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.