I believe the issue is whether our civilization will survive without marriage and the family as its cornerstone. In my lifetime there has been a concerted attack on our primary social institution which, if marriage is destroyed, the family’s disintegration will accelerate and the “atomization” of society will increase. That, or polygamy will become legal (it’s the same argument) and women will have lost thousands of years of slow and painful evolutionary progress. Either way, women and children are the losers.
We can grant those legal advantages to homosexual couples without destroying the institution of marriage by granting them marriage. Polygamy will be next and then our civilization will be completely destroyed. But, I suppose, that is the point.