"REGRETTABLE NECESSITY, n. An avoidable atrocity. The term is often employed by presidents and prime ministers when announcing bombings of civilian targets and invasions of small countries."
In addition to that, up to 8 to 10 million children of gay parents and same sex couples would benefit from same sex marriage. Can you argue anywhere near that number of children would benefit from incestuous marriage? Polygamous marriage?
Trying to oversimply this argument isn't going to work. The parental evidence and number of children who would benefit are factors supporting same sex marriage, and there is no such parental evidence or number of children who would benefit from recognizing other forms of marriage like polygamy or incestuous.