In the sense that almost anything can lead to murder, he’s probably right.
In the sense that legalizing same sex marriage will lead to polygamy, incest, and bestiality, he’s dead on right.
No matter what contortions they try to use, they cannot get around the simple fact that once you break the 1-man, 1-woman = marriage paradigm, you have effectively gutted the possibility of any limitations at all. If gay marriage is a valid lifestyle choice, there is absolutely no reason why 2 (or more) women and 1 man can not be.
And has been pointed out numerous times, there is far greater precedents for polygamy and incest than there ever were for gay marriage. In fact, I have yet to run across ANY historical precedent for gay marriage. The fact that homosexuality was condoned, or even celebrated in other cultures does not mean that it was officially sanctioned.
If someone knows an exception to that, I’d love to hear about it.