Hmm, I have two thought processes on gay marriage (pertaining to this topic at least).
1. I personally have nothing against gay marriage and in the same vein, I have no problem whatsoever with multiple wives/husbands. If you are going to enter a ridiculous contract like marriage, then anyone should be able to do it with as many people as they want. I find the whole thing pointless and nonsensical, but to those who want to do it there should not be restrictions except for common sense terms (animal marriage for example).
2. His thought process is not exactly wrong. For example look at gun control, the SOPA bill, and other things that are continuously voted against not necessarily because it is DIRECTLY damaging, but mostly because it opens a gateway into new laws in the future, it is a line crosser that opens up voting for other restrictions/changes that would never have happened, potentially bad ones.
I do not agree with him on this subject because I believe limitations for certain groups of people to be wrong (in most cases), and if 4 guys want to marry 2 women I really couldn't care less, or if two gay want to or whatever, I simply don't care, it's your life do what you want with it so long as you are not imposing on others. However, his logic and thinking is not "wrong" in a general sense, yes passing a law like this DOES open up more groups of people to want their marrige rights to be included, and yes it MIGHT make groups of people/relationships who otherwise would never ever be allowed to marry, step up and start making this same drive towards equality. Many people seem to not enjoy this idea, why I do not know, but they do.
Secular gay marriage (if you demand legal church weddings then you are imposing on other people's religious belief/system, you are no better than those demanding on your rights) should be perfectly legal for anybody of any quantity that wishes to do so, but his logic is not necessarily incorrect in that yes it will and would open up doors in the future for more groups that people probably find strange/odd/whatever to want their piece of the pie.
Nothing is ever black and white and even though it sounds stupid, not everyone who is against gay marraige is "anti gay" or hates gay people or are homophobic in any way, many of them realistically do not agree with ANY change in marriage, be it gay or otherwise, and I think that is where he is coming from, to which I say... that is your opinion, sir.