Yes. All humans should be protected under the law.
No. Marriage is between one man and one woman, period.
No. Only homosexuals and heterosexuals should be allowed to marry.
I donít care what they do as long as they stay out of my business.
My catís name is Mittens.
Abuse is abuse and I am in no way trying to down play it. But abuse happens in all kinds of currently accepted domestic arrangements, from living together, to monogamous marriage to divorced to single parent. Polygamous marriages is/would be no different in that aspect, but likewise it is true that all the successful traits would occur as well.
Incest usually occurs in abusive households where child molestation is occurring, unlike "gay" sex or "interracial" sex. Most cases of incest are between father and daughter, so it's easy to see how a father could "groom" his daughter into marrying him and making it a legal thing if incest were ever to be legal. Incest and being attracted to your family members is a sign of mental illness.
Gay sex and interracial sex have ALWAYS happened and have not always been socially unacceptable in all cultures. The same cannot be said of incest, which is taboo for a good reason. Anyone who can't understand that should speak to a shrink ASAP.