The “gay rights movement” and the push to legalize gay marriage originated with the feminist movement of the late 60’s and 70’s. If anyone is old enough to remember (like me!) their original platform was based on the notion that marriage for a woman was akin to prison or a concentration camp. It has been a militant attempt to convince women that they don’t need men and don’t need marriage. These same feminists, which unfortunately included many hood-winked men, ushered in the dawn of the gay rights movement. None of these activists are interested in “joining” an institution which “hates” them. To clarify that point: The Church does not hate - nor does the government. The positions we hold as believers are based on love of God and fellow man. The position of the state is to bestow benefits and recognition upon traditional marriage because that is where new “little citizens” of the state will come from.
The gay marriage obejctive is not to “join”. The objective is to overturn tradition. Most activists are also pro-open marriage, multiple partners, polygamy, etc.