The good folks on the Left may have thought that their popular fight for so-called "marriage equality" was solely on behalf of homosexuals.
However, as with many liberal causes, the floodgates have been inadvertently opened (or, as some might call it, the slippery slope has been greased even further) to legalize marriages between various other alternative relationships--including several that some on the Left may object to, while others on the Left may desire.
I can't think of a single liberal emotional appeal, or even legal argument, brought to bear in forwarding the cause of same-sex marriage, that wouldn't also apply to polyandrous or polygamous or incestuous relationships. With the exception of the matter of "consent," the same holds true for pedophilia and bestiality, if not also marriages to plants and inanimate objects.
And, if the good folks on the Left were genuine in their unqualified belief in and support of "marriage equality," then consistency demands that they support legal marriage for those other alternative relationships as well.
It is uncertain whether legalization of these alternative marriages will happen anytime soon, though efforts are already under way to legalize polygamous and polyandrous and incestuous marriages as well as marriage of humans and pets and between pets. (See HERE and HERE and HERE and HERE and HERE and HERE and HERE and HERE and HERE and HERE and HERE and HERE and HERE and HERE and HERE and HERE )
The floodgates were opened in more ways than just marriage. Legalized same-sex marriage cleared the way for the loosening of other social morals. Again, reports are also coming in that pedophilia is on the rise after legalization of gay marriage (see HERE), and support is mounting for legalization of pedophilia. (See HERE and HERE and HERE and HERE and HERE)
For an explanation as to why these same-sex marriage Leftist LUNCs have happened, see: The politics of Victimization, Compassion, Compassion, Equality, Emotions, Bullying, Propaganda and Disinformation. [the unlinked topics will be posted
later as they are completed]