Except there has never been any obligation for married couples to procreate and social conventions, much less laws, aren't dictated by base biological urges.
Marriage has, first and foremost, always been a business arrangement. Even though the idea of love is a factor in most cultures now, that is something that has come about in recent centuries. Dowries, arranged marriages and the concept of spouses were prevalent far before that. Even within religious-based unions. Even now, for all of the strands of love and devotion connected to it, marriage still seems to be more about financial stability than emotional stability.
That's why polygamy and gay marriage are not comparable. A marriage between two men , two women or a man and a woman all legally operate the same. If a man is married to several different women and he dies, are those women still married? If those women gave birth, do the children belong to all of the women equally or do they only belong to the respective woman who gave birth to them? What if the husband falls into a coma and has no living will? Which of the wives has power of attorney over him? If the women are employed, do they include the other wives' children on their health insurance? Are the women's tax obligations contingent on the number of children to whom they gave birth or the cumulative number of children in their family?
A polygamous marriage is an actual redefinition of marriage. A gay marriage isn't. There is no "slippery slope". It literally is just an inclusion of a group that had previously been arbitrarily excluded.The concept of two people creating a familial structure inherently makes sense, regardless of the gender of the people involved.
