There's plenty of verses telling us to obey governemnt laws if they don't
conflict with God's law. As far as an absolute moral aspect, it seems like
polygamy didn't work well in all examples discussed in the OT-one wife
favored over the other, quarrels among wives or children, etc. However, in
the case of evangelizing a polygamous culture, it seems that polygamy is
better than divorce (particularly if wives are dependent on their husbands
in the culture - non-favorite wives become outcasts).
There's plenty of biological evidence favoring polygamy as a normal
condition for hominids, including the widespread acceptance of the
practice, strong sexual dimorphism, and females being smaller and younger
at maturity. Also, gorillas have harems.
Although "evolutionary predisposition" is often claimed as a
justification for various sexual behaviors, it is usually for something
more socially acceptable than a harem. It also relies on the assumption
that if it can be justified evolutionarily or blamed on one's genes, it is
OK. However, many things can be justified by these criteria yet are
considered wrong by people who invoke these standards (e.g., rape or racial
discrimination).