I don't think that the "gimmick" is limited to just liberals. It is doubtful though that your logic will be successful in preserving marriage because your own agenda gives you away.
We'll see in November, although I, too, have doubts about this CA amendment passing in one of the most liberal states in the U.S. After all, the voters elected the legislature that twice tried to legislate same-sex marriage into existence.
The institution of marriage has many more enemies than just the encroachment from the homosexual left: Voluntary single parenthood springing from culturally acceptable promiscuity; easy, flippant divorce; adults that are increasingly mentally and emotionally immature, thinking and acting like the children that are in their charge; liberal notions of what a family is and should be; the radical feminization of society. Perhaps radical homosexualization is but one branch of this rotten tree, but is just as destructive as all the others.
... because your own agenda gives you away.
Agenda, huh? And the left doesn't have an agenda that's as clear as crystal? And which "agenda" (value system, actually) does the average American identify with? Voters in 27 states to date have amended their constitutions by referendum to protect marriage as the one man/one woman institution it has historically been. Several other have passed laws short of constitutional amendments defining marriage as only between a man and a woman. These issues have passed by significantly wide margins wherever they have been put to a public vote. You're assertions don't line up with the facts. Do a little research next time before replying.