You have one and a half good questions here. I’ll take them in reverse order.
Transspecies unions? Gimme a break. Can your dog sign a rental agreement, or a business contract, or get a car title transferred into its name? If not, why on earth would you imagine it could enter into a civil union? (Before you ask the obvious next inflammatory question, the same answer applies to having minors enter into civil unions). This is a bad question.
Incestuous unions? Here’s your half-right question. Incest refers to sexual relationships, and civil unions don’t–or shouldn’t. That’s the bad part of the question. The good part is whether I’d allow close blood relatives to enter into civil unions with one another. Lemme give you an example, one I’ve used in previous threads:
*Bill and Amy are siblings. They grow up, move to different states. Bill gets married, and two years later, Amy gets married. Neither one has children in their marriages, for various reasons.
Fifty years pass, and Bill’s wife dies, and then Amy’s husband dies. Neither one has any family besides one another. Then Amy develops a painful, disabling disease. Bill, who loves Amy like a sister, invites her to come live with him so that he can provide her the care in her waning years that she needs. She does so, and they apply for a civil union, so that they can simplify their finances and their various rights.*
Why on earth should Bill and Amy be denied a civil union? I’d absolutely support issuing one to them.
Finally, your completely good question: should polygamous marriages get civil unions? I say no, but not for any moralistic reasons: I simply say no because it’s too complicated. The vast ajority of people end up in a relationship with one partner, and there’s a unique bundle of rights, as mentioned above, that would be issued toward that partner. Polygamous marriages are so different from one another that there’s no one-size-fits-all plan that’ll cover them, and certainly they won’t be covered by the civil union bundle that covers two people. Remember where I said efficiency was one of the two criteria to keep in mind when designing civil unions? Polygamous relationships are exactly why I put that criterion in.
Let’s turn it back to you: would YOU deny Bill and Amy a civil union? Why or why not?
Daniel