I don’t think polygamy is next, because expanding (destroying) the definition of marriage was but a method to get to what they are truly after -
criminalizing Christian beliefs.
They’ve achieved that with “gay marriage”, so there’s no need to go further on the marriage front.
I look for trannies to be the next tightening of the screws. Make it illegal to “discriminate” against transexuals. You have to let them in your daughter’s lockerroom at school, in the bathrooms, showers, etc, or you will be sanctioned by the state.
Also, start looking for big pushes to remove 501c3 status from anything Christian.
After that, the jailing of pastors for preaching anything “homophobic” or “transphobic”.
Once you legalize one perversion, the others will rapidly follow. They will rule that you can’t discriminate for one and not the others.