As conservatives we should have asked this question when the Supreme Court ruled in Lawrence v. Texas. At that time the asnwer to your question was polygamy, and we we were only slightly in trouble.
Now unfortunately, the answer to your question is ployamory, bestiality, canibalism and necrophilia. Since adult consent can be obtained or implied in all of these cases, our enlightened liberal friends see nothing wrong with them.
Getting "consent" in the case of necrophilia might be a tad difficult, even implied. Unless it was obtained pre-decease. I don't think we want to go there. In fact I'm really SURE we don't want to go there...