I think it is a close case. Most of the same arguments work (notably, and importantly, the same arguments do NOT apply to pedophiles and bestiality, due to the lack of consent of one partner).
There are some distinctions. For example, throughout history polygamy has been closely associated with second-class citizenship, abuse, and economic hardship for females involved in these relationships. There is also some modern anecdotal evidence about harm to children and coerced marriage in religious groups practicing polygamy (Warren Jeffs). Hard to say whether that is representative of all or most or even a meaningful percentage of polygamists. I doubt there is much reliable statistical evidence on how women and children fare in modern polygamous relationships - very small sample sizes I would think.
I think it is a harder case to make on an "equal protection" argument. Dan+Joe = Stan+Mary is an easy argument to make - it's all the same except for what they do in the bedroom. Stan+Mary = Dan+Jane+Jill is harder. Brings up all kinds of really difficult issues around inheritance, equitable distribution of the marital estate in case of divorce, etc. There are tons of legal assumptions and default settings in family law that are based on a two-adult family unit. Dealing with legal polygamy would require a pretty big overhaul of state family law. Dealing with same sex marriage just involves a change in pronouns. Because of all that, a polygamous couple will have a hard time saying that they should have equal protection of law - it's more like special protection.
ETA: laughing at my choice of words "polygamous couple". What's the right phrase? Polygamous group? School? Pride? Herd?