Gay Marriage - Yes because there is no logical reason why it shouldn't be allowed. Polygamy - Yes but only under certain circumstances, with limitations regarding marriage. Incest - Yes and No, it should be allowed if it's between consenting adults, but there is a risk of children being born with genetic defects from this type of relationship which is why I usually cannot decide with this one.
I feel as though they're not quite comparable. As briantheliberal said, within certain restrictions, all of these should be allowed, it's just a matter of specifying. I find it hard to vote things like these.
I would disagree with polygamy. The inheritance laws for divorce would be difficult and other laws regarding a group of people marrying each other would be difficult to create. Gay marriage is the only one that should be allowed and only because its between two consenting adults that causes no ill harm or legal difficulties