This is too much of a general view. Kings having numerous wives are not always true. There are other issues to consider.
Most cases. King used to have only one Queen, the one whom he is married to and who bears the child for the throne. The rest is bit different story. There used to be harem, where a king would have dozans of women, he is not married to anyone, but sure most kings loved polygamy. There were not much things available to keep people busy, no TV, Movie, Smart phone, Internet or porno wasnot there. The pleasure one would seek was having sex.
On your question was polygamy the sole reason, yes and no. Polygamy was for pleasure, but also a lot of new girls were added when they won a war and capture the defeated king's harem girls. Its like a pride to have a large harem, and king wont have sex with everyone everyday. He would spend time on naked bodies, that was a pleasure.
Yes its okay to have polygamy in modern time too. Rather than start all cheating and other facts, I dont see why a man wont have sex with multiple female or a wife with multiple male. If a couple are not satisfied with one another, either they should have open sex life with consenting adult and protection, or suffer with a sex less life. The main issue here is one shouldnot do anything where the partner is deprived of something. If one partner is not capable to satisfy his/her partner, why the other one should keep suffering? Polygamy is within our gene, we try to curb it or we do curb it, but if this was open, noone would bother to stay in a single marriage, no matter how great the love is.
Thats my opinion, and opinion is like an asshole, everybody has one. Oye