Polygamy (17%): Unacceptable
Cloning Humans (14%): Unacceptable (slippery slope reasons)
Extramartial Affairs (9%): Unacceptable. No exception for consensual affairs / polygamy.
But why?
Not the OP, but polygamy is generally associated with a lack of women's rights.
The thing about consensual affairs makes it clear that it isn't just that association.
I just don't like pretending that the polygamous, polyamorous, and open relationship people are normal or acceptable, or that what they do qualifies as marriage in the Western sense even if they have a marriage license.