My view is that marriage is a religious entity, and the church alone should be able to decide who it wants to marry. However, the state (being separate from any religion), should be able to give the same legal rights that they now give for marriage to almost any two people who want to be married. Call it a legal union, or whatever else you want, but the rights should be there and be independent of any church.
The state shouldn't recognize polygamy and incestuous relations. Excessive polygamy shouldn't be condoned because then there won't be enough of a particular sex to go around, and incest shouldn't be allowed because it leads to biological problems. Basically, for logical and scientific reasons.
You may then wonder why allow gays to marry, since it could also lead to population imbalances (if for example more men are gay than women). The reason for this is because they can't help being gay (believing otherwise is idiotic), and no matter what the law is they are going to pair off this way anyway. Polygamists may marry under their church anyway despite the law, but this is different in my mind, because nobody has a condition where they are attracted to ONLY multiple other people.
You may then further wonder if I would support unions that don't lead to population imbalances and genetic problems, for example four bisexuals (two male two female) in a union... well I personally wouldn't have a problem with it, but we'd want to make sure they wouldn't have an unfair advantage over others under the law, I would think.
In summary, provide legal opportunities for couples that make sense, and let churches perform whatever ceremonies they want to please their respective gods.