Same argument 120 or so years ago:

"If dem ni**ers can get married, folk may start marrying cows now for alls i know"

Basically, there is a fundamental difference between consensual adult humans getting married and examples you listed.
By the way, i for one am not too much against polygamy either, but polygamy is more of a whim or preference, while homosexuality is a hard fact you are presented with when you discover your sexuality.