The next thing is legalizing pedophilia like they have in Holland.
Sex with animals is the next thing after that.
But otherwise, he's right on the money.
There are a few intervening steps between Same Sex Marriage and Animal Marriage:
Group Marriage
Sister Marriage
Grammar School Marriage
Daughter Marriage
After that comes Animal Marriage. So technically, the gentleman is wrong. Group Marriage is next.
Both San Francisco and Salt Lake City have advocates agitating for Group Marriage. So the slope down to Animal Marriage is before us, and one must wonder how far we will slip.
I think bigamy will come along at about the same rate as 12 year old age-of-consent.