I support none of those.
Polygamy for the reason that a marriage is a union between two people, not multiple. There would be no benefit for society as a whole, and in fact the bureaucracy of changing all forms, legal precedents, old convictions would need to be turned over(ex post facto only works one way)would cost an astronomical amount. It would be a mess. Nothing would really change if same sex marriage were to be made official.
Incestuous for the reason of genetic defects are greatly increased, and for humanity to survive, there needs to be gene diversity. Government forms would also need to be changed to include any possible combination of relation relation union. (Don't even try and get into the "homosexuals can't reproduce" debate, yet, that will come)
Bestiality, that is really a no brainer, marriage is between two people. Also, there cannot possibly a mutual love or respect of the type marriage commands, as one is an animal, that is not self aware, or able to communicate on the same level, and the other is sentient.
Pedophilia, again, a no brainer. Children are incapable of making rational decisions until after puberty is complete. Legally, no person under the age of 18(with rare exception) are able to enter into any binding contract. This includes marriage. The exceptions to this are the Delayed Entry program for the military, where a 17 year old can sign a contract that is binding to enter the military upon turning 18 and finishing school. Attending school is another contract that is binding to a minor, although that is not one signed by them, but legislated.