Polygamy is okay, bestiality isn't. Consent is the limit, as long as everything is between consenting adults, I'm fine with it.
As for murder... give me a break. Yes, murder is illegal because it is morally wrong. No, that is not the only reason murder is illegal.
From a purely economic standpoint, no buisness will want to invest in a country where its employees could be killed at any time with no repercussions. Heck, the entire economy would collapse, weapons- not money, would become the basis of power.
Essentially, there are some objective, logical reasons to outlaw murder. Nobody has been able to provide such for gay marriage.
