Laws change to fit the needs of its citizens. There have always been gay people. They were marginalized or misunderstood in the western world until basically the end of the 20th century.
With access to more education, more wealth, changing attitudes towards sexuality, and better forms of communication, existing gay people were able to find each other and form communities. They banded together to fight for rights they had always been denied. They organized, worked hard, got the word out etc. They protested, got killed, got spit on, got beaten, and finally started to get respect. Because, they didn't go away.
They won the hearts and minds of an increasing portion of the population.
Regarding the slippery slope towards imminent dog/human marriages: where is the community that wants that? Will they be willing to band together, struggle, win over those different from them? Where is the historical record of human/dog relationships and romance? I just set that aside as ridiculous.
As for polygyny. Same thing. Worry about it when there is actually a large portion of Americans who embrace that lifestyle. The government of a country exists for the people, and the laws ought to reflect what works for the people that live within it.
That?s what just happened Wednesday.