Where does it stop

When gay activists argue we should change the definition of marriage that often leads me to think where this slippery slope will stop.

Current definition of marriages
union between 2 adult human beings of opposite sex

the version of marriage definition proposed by gays is to remove the clause "opposite sex"
union between 2 adult humans beings

The polygamists will then also argue to change the number 2 to "x" to permit polygamy.
union between x number of adult humans beings

Then under age marriage proponents will argue to drop the word "adult"
union between x number of humans beings

The bestiality sickos will then argue to drop the word "human"
union between x number of beings

and so on so forth.......if we allow gay marriages to go ahead then I ask "where does it stop"?