Actually some lady in the UK recently tried to marry her cat.

But, the next "civil rights issue" after gay marriage is fully normalized will then move on to legalizing polyamory and polygamy. After all, "who is to say" multiple people can't love each other?

After that .... hard to say when, but NAMBLA will start to say "...we want to be considered normal too" (my paraphrasing.)

North American Man/Boy Love Association - Wikipedia, the free encyclopedia