Judging by the divorce rate [not to mention the numbers of infidelity, whether known or un], the institution of marriage isn't in real great shape to begin with. But I don't see where any two people who wish to make the commitment are a threat to marriage - more like 'imitation is the sincerest form of flattery' innit?
The threats to marriage are elsewhere, mostly [IMO]: unreasonable expectations, and the inability of people to cope with stresses in a psychologically healthy way. Life gets tough, some people comfort themselves with drugs, alcohol, food, adultery - whatever makes them feel better temporarily. Defense of marriage would be better accomplished by better preparing people for the good and bad. [And decreasing unemployment, which is a major stress factor!]
As Bob Dylan noted [decades ago, lol] the times, they are a changing - just this year, 2 more states have approved of gay marriage, because the voters want it.
The 9 states that allow it [Ma, Ct, Ia, Vt, Ct, NH, NY, Me, Md, Wa] are outnumbered, but the wheel is turning, and more and more see it as a matter of civil rights.
I think that encouraging stable and committed and monogamous relationships is a positive step forward in our cultural evolution.