Your takeing the position that group marriage is not more legally complex than samesex marriage, but you have had little to say about the reasons that I outlines that suggest that it is more complex.
Since you support plural marriage, you should have answered those questions for yourself.
It doesn't seem so complicated to me. Inheritance law is more complicated when there are more than one heir, but we don't use that as a reason to ban plural childbirth. If it really is such a brain teaser for you, here:
How would employer benefits be handled? If you are married to 5 people, would an employer be obligated to extend benefits all of your spouses?
The libertarian answer: However the employer wants to handle it. Employers shouldn't be obligated to extend benefits to anyone.
If an employer does extend benefits because the law forces them to, I see no reason why they can't do that for multiple spouses, as they already do for multiple children.
If one of you numerous spouses has a child, are all of them presumptive parents?
The family could choose to designate only two people on the birth certificate, or three or more as in the example of three dads that you seemed to have no problem with.
Would the IRS be able to deal with a joint income tax return filed by 10 people?
They can't deal with my very simple one spouse joint tax return this year. It has bounced back three times with no real explanation of what the problem is, no one answering the supposed customer service line, and no help from the website, even though I sign on to my account. If we have to wait for the IRS to suddenly become competent, we'll wait a long time.
If someonewants a divorce, do they have to divorce everyone or can they be selective?
In both the Morman tradition and the Muslim tradition, the woman marries the male, who later marries another woman, and so on. Each of the women are married to the man, so any divorce would also be between the man and the woman. Morman women call each other "sister wives," but as far as I know, do not consider themselves married to each other.