Civilly, the essence of marriage (whatever else it may be, which many of us would agree contains a great deal of “whatever else”) is a contract between two people creating a theoretically lifelong partnership of all that they are and all that they have. (This does not automatically create community property or dominance of one over the other, lest my wording be misunderstood.) It is easy to describe the relationship created in such a binary partnership, what happens when it is dissolved, etc.
To extend this to same-sex couples differs in very little from changing the age of consent from 18 to 17. “'Two persons of opposite sex not already married, not within prohibited consanguinity, who have reached the age of 18 years, may contract a legal marriage within the state of West Colozona.” Change 18 to 17 to make the latter change, strke “of opposite sex” to make the former.
On the other hand, creating a relationship between three or more people requires extensive changes to existing law. If two women are married to the same man legally and he dies, do they remain married to each other, or is his death the dissolution point of the marriage? If so, what if it’s one of the women who dies – does that dissolve the marriage of the survivors? Does whether the women are sexually attracted to each other enter into the picture? If so, why? If a marriage of four men and three women is dissolved, who has custody of the children, and why him or her or them?
This is not to argue against legalizing polygamous unions, but to show the difficulties that can arise.
To me, that makes a clear distinction, a level spot on the slippery slope. Legalizing the union of two people previously barred from engaging in marriage with each other is simply done, and requires very little revision of the laws. Creating a marriage relationship for a polyfamily creates much more complexity, and requires some detailed thinking through of how the laws should deal with that sort of relationship.
Short note to Kanicbird re the post above: I agree that marriages should support families and provide for progeny. I wonder if it will come any clearer from me than from others who have said it in the past: many gay couples in fact do have children, either from previous heterosexual events (not necessarily relationships, much less marriages), from chosen in vitro fertilization, or from adoption. We have a member here, Seven IIRC, who is the (straight) son raised (very successfully, from his account) by two fathers. Frankly, my dear, that dog don’t hunt.