Good position on the we are all mormon thread. Makes me think.
here's a point that may change how you feel:
In the first place, by its very nature, marriage has always been, and currently is a socio-religious, heterosexual institution; consequently, when the word marriage is used, it always has, and currently is indicating only a socio-religious, heterosexual institution. So knowing, there is no need to over define marriage by referring to its institution as "heterosexual marriage."
With that out of the way, marriage itself came into existence BEFORE the establishment of governments. Marriage itself is a socio-religious institution. After marriage, then came the family as a product of marriages, which both marriage and families came BEFORE the establishing of governments.
Much like the right to bare arms was indeed in place and an accepted fact in American BEFORE the drafting of the Constitution or Bill of Rights, marriage was also in place before the drafting of these two documents. The history of marriage is unequivocal. No one was trying to define marriage as a definition wasn't needed. The only reason amendments are being drafted nowadays is because of homosexuals that are trying to actually RE-DEFINE marriage.
The homosexuals are the one trying to create a new definition for marriage, but more importantly, the government officials also understand that the government has no right to redefine an institution that existed before it did.