You can't equate actions taken during biblical times to marriage practices today.

It's true King David and King Solomon had many wives and concubines.

You have to understand that during this period of time in Jewish history, the actions taken were political in nature and built around creating stability for Israel. Marriages and producing children with foreign kings or competing powers helped alleviate political and military tension. It was an act of unifying people and ensuring future survival.

So much of Jewish history is a chess game. You have to know when the battle is worth fighting vs the need to play politics and seek out the peaceful solution.