Well, it still is in some parts of the world:

KUALA LUMPUR — Rohaya Mohamad, 44, is an articulate, bespectacled medical doctor who studied at a university in Wales. Juhaidah Yusof, 41, is a shy Islamic studies teacher and mother of eight. Kartini Maarof, 41, is a divorce lawyer and Rubaizah Rejab, a youthful-looking 30-year-old woman, teaches Arabic at a private college.

The lives of these four women are closely entwined — they take care of each others’ children, cook for each other and share a home on weekends.

They also share a husband.

So, should polygamy be legal?  To address this question, I think it is useful to consider two prior questions:

Should government "supply" and enforce the particular bundle of contracts known as marriage?

If it does so, should it restrict this supply to opposite-sex couples?

My answer to the first question is no: government should establish and enforce default rules about the division of property from communal living arrangements, about inheritances, and about guardianship of children, but it need not and should not bundle these rules into the particular package known as marriage. 

My answer to the second question is also no.  If goverment is going to supply marriage, it should do so in the most neutral way possible.  This means treating same-sex marriage and polygamy just like opposite-sex marriage.   Government should calls these contracts civil unions, leaving marriage to religious institutions.