For one, it helps save marriage from the fate of an anachronism, which is a good thing. Instead of marriage being particularistic thing that only heterosexuals couples aspire to, it becomes what it is most powerful as: a NORM.
It brings gay families and relationships that ALREADY exist and will continue to, and includes them in society instead of forcing them to develop and function outside of it to some degree, which creates splits, dissension, and competition for social legitimacy.
For starters. But Rauch’s book makes the case at even greater length and detail, and with more convincing language than I can supply.
http://www.amazon.com/exec/obidos/ASIN/0805076336/qid=1100231461/sr=2-1/ref=pd_ka_b_2_1/104-0524418-8760714