In defense of marriage between a man and a woman, I have no qualms about explaining how obviously man and woman are designed that way, that the family is the fundamental social unit and that its mission is clearly procreation (and other things too), that a child needs both a father and a mother, stability, etc. For more thoughtful stuff, there is plenty of material in the teaching of all world religions.
Of course in the 12 Steps, people do have some qualms about this approach and in SA there have been efforts to find other motivations, but why bother? The really big guns, the sure-fire way to support the man-woman definition, the stance understood and still supported by the large majority of the human race, is the sort of stuff I have mentioned above.
In the development of SA, in my opinion, there has been de-emphasis on the development and promotion of families including having children, in favour of an emphasis on the individual's lust, sexuality and recovery. It is hard to defend the man-woman sobriety definition within this framework. However the fix is simple and will be an unavoidable consequence of our Fellowships' journey toward deeper sobriety and recovery - just a shift in balance and emphasis to include and promote sex's appropriate and healthy role in family life and procreation.