Nope, it’s a fair argument.
However, your “paternity” argument is messy, as in an opposite scenario (F/M/M/M/M) or a diverse one (M/F/M/F/M/F), it becomes increasingly unclear as to who the father might be without tests.
It is complex in terms of the socioeconomic and legal topics it introduces, but I understand why some people people might want polygamy legal nationwide.
For example, Stacy, Susan and John all went to school together. The graduated high school and all received degrees from the same college.
They know everything about each other, love and care for each other very dearly (and equally). What happens when they realize they are all in love with one another? Do just two get married, and the other one is denied the privilege of taking part in the wedding? Why do they have to even think of making that choice?
So I get how it might be compared to gay marriage, with regards to ethics and morals. Still a bit of a stretch, in my opinion – most polygamist unions aren’t like what I described above.