The slippery slope argument was long used as an excuse to deny marriage equality.
My own point of view is, I don't know the practical details. Would there be a cut-off to how many people could be in a marriage? 3 spouses? 10? 20? And sorting out the genders, could a marriage be multi of each gender? How would that work for taxes? Liability? Inheritance? Would an employer be required to cover multiple spouses for medical benefits? Would there be an increase in immigration marriages? How would a divorce be sorted out?
Those are just questions. Not expressing any kind of disagreement with the concept. I don't see anything morally or ethically wrong with expanding marriage to include multiple consenting spouses. We only live once, and we should have a choice in how we live, as long as there is no abuse and people treat each other with kindness.
I don't know if I agree that LGBTI marriage equality automatically leads to plural marriage equality.
"Polyamorous people and couples I should mention are also demonized by fundamentalist religion much like like lgbt individuals as well as romantically and sexually active same-sex couples are."
What ISN'T demonized by fundamentalist religion? When they have the brazenness to attack a purple, purse-carrying teletubbie, there can't be much remaining in their twisted world deemed worthy enough to be acceptable.
You ask "what ISN'T demonized" by fundies? How about crazy-ass politicians who invent terms like "legitimate rape"...and Michelle Bachmann, who bashes needy people who can't survive without SSI and MediCare, but takes humongous tax breaks for acres and acres of idle farm land that her family owns....and the Catholic bishops who are wetting their diapers about having their insurance companies provide contraceptive prescriptions for their non-Catholic employees. (Insurance comanies DO pay for Viagra and Cialis...is something wrong with this picture?)
I don't see anything wrong with plural marriages no matter what gender(s) are involved, as long as the parties involved are all consenting adults. (Read Robert A. Heinlein's novel The Moon is a Harsh Mistress for intelligent discussions of different types of group marriages.)
I have always believed that the Mormon Church's hysterical (and monetary) support of California's shameful Proposition 8 was their revenge against the 19th century condition that Utah statehood depended on the state constitution banning polygamy in the 1890s. (Many Mormons still believe that the ban was contrary to the First Amendment.)
Personally, I think people who believe the Book of Mormon has anything to do with truth are living in a fantasy world, but then so are buybull believers. There is nothing anywhere in the buybull forbidding multiple wives...and concubines. Islam allows as many as four wives. But, of course no holey book says anything about a woman having more than one husband. Not FAIR!
If a particular group marriage works, and everybody is happy, fine! Legalize it. (Beats the "bleep!* out of the fake...and miserable... Hollywood marriages that were cooked up to prevent the movie-going public from finding out that a secksy leading man, or woman, was gay.)