patrickaa317 wrote:I would rather focus on finding someone that supports gay marriage but does not support marriage involving three or more people
That would be me, though I could change my mind.    First, I want to put forward that a lot of unmarried people today are essentially polygamist/polyandrists.  The problme with unmarried people having multiple kids from different partners  is partly the same as with polygamy/ployandry, but not entirely.    
I will start with the difference.   The difference is commitment and, usually, consent.   (For this debate, I will exclude the Warren Jeffs type scenario, becuase I believe that is an aberration.   Kids should not be engaging in sex.   Most of those actually wanting legalized polygamy see that as abhorrant as well).  That makes a big difference, since the fundamental historical reason for marriage is "legitimacy" of children and securing inheritances.    Polygamy/Polyandry do that as well as any other kind of marriage.  However, biologically, there is a huge difference in Polygamy.    One man having a hundred kids is, biologically and socially very different from one man having 12- 20 kids from the same woman.   Today, there is some question emerging on whether having that many kids from one woman is really a moral choice.  (note, I said, "there is a question/debate" on this and that it is "emerging".. I am NOT saying I have this opinion!).   That is still an emerging debate with many complications, but society has already declared a problem with polygamy and the idea of the power one man can have by having 100 kids is very much a part of the issue.  
This multiple children bit is also part of the problem with umarried individuals, though in that case, it is often tinged with a failure to support or take responsibility for the progeny.  
Interestingly, polyandry would not pose this problem, but is more universally rejected.  I think that gets into the roles and views of women.   Many anthropologists point to the fact that while men absolutely require women to have and raise children, the need is nowhere near as great in the reverse.     The needs that are established are historic, based on the need for protection and security.. and those needs are not necessarily the same today.  That may be  part of what can make polygamy now a more viable option, ironically.  However, the fact that more men can easily support multiple women now does not negate the problem of multiple births.