As is demonstrated above their is clearly much more correlation between incest marriage and same-sex marriage. In fact it is word for word identical.First off, why do you keep mentioning bestiality? That would be a legal contract between a human and an animal, and that is not allowed for any contract.
So you could discriminate based on gender except for special laws passed preventing it. Sounds like that is an argument in favor of being able to legislate that marriage is between opposite gendered people.To clear things up for you, the following are types of marriage that you've mentioned and the basis they are discriminated on:
1) SSM: Gender
2) Incest: Familial Relation
3) Polygamy: Marital Status
4) Bestiality: Species
Only one of those things has ever been brought up in civil rights legislation: gender. It is not illegal to discriminate based on any of the others. That is why the situations are not comparable.