If sex is to be restricted to a husband and wife, why did Solomon have hundreds of concubines?

This account is recorded at I Kings 11: 1-8

There in short it states, "And he came to have seven hundred wives, princesses, and three hundred concubines".

Your answer is that was both these foreign wives and these foreign concubines that exposed him to a real "mixed stew" of pagan thinking. Can you imagine trying to keep all of those different women happy? He must have compromised his faith trying to get along with them, trying too hard to be agreeable. He allowed his personal relationship with his God to suffer.
Now for some more detail:

"As long as Solomon remained true to the worship of Jehovah, he prospered. Evidently his proverbs were uttered, and the books of Ecclesiastes and The Song of Solomon, as well as at least one of the Psalms (Ps 127), were written during his period of faithful service to God. However, Solomon began to disregard God’s law. We read: “And King Solomon himself loved many foreign wives along with the daughter of Pharaoh, Moabite, Ammonite, Edomite, Sidonian and Hittite women, from the nations of whom Jehovah had said to the sons of Israel: ‘You must not go in among them, and they themselves should not come in among you; truly they will incline your heart to follow their gods.’ It was to them that Solomon clung to love them. And he came to have seven hundred wives, princesses, and three hundred concubines; and his wives gradually inclined his heart. And it came about in the time of Solomon’s growing old that his wives themselves had inclined his heart to follow other gods; and his heart did not prove to be complete with Jehovah his God like the heart of David his father. And Solomon began going after Ashtoreth the goddess of the Sidonians and after Milcom the disgusting thing of the Ammonites. And Solomon began to do what was bad in the eyes of Jehovah, and he did not follow Jehovah fully like David his father. It was then that Solomon proceeded to build a high place to Chemosh the disgusting thing of Moab on the mountain that was in front of Jerusalem, and to Molech the disgusting thing of the sons of Ammon. And that was the way he did for all his foreign wives who were making sacrificial smoke and sacrificing to their gods.” 1Ki 11:1-8.
While this took place “in the time of Solomon’s growing old,” we need not assume that his deviation was because of senility, for Solomon was relatively young when taking the throne, and the length of his reign was 40 years. (1Ch 29:1, 2Ch 9:30) The account does not say that Solomon completely forsook the worship at the temple and the offering of sacrifices there. He apparently attempted to practice a sort of interfaith, in order to please his foreign wives. For this, “Jehovah came to be incensed at Solomon, because his heart had inclined away from Jehovah the God of Israel, the one appearing to him twice.” Jehovah informed Solomon that, as a consequence, He would rip part of the kingdom away from him, but not in Solomon’s day, out of respect for David and for the sake of Jerusalem. But he would do it in the days of Solomon’s son, leaving that son with only one tribe (besides Judah), which tribe proved to be Benjamin. 1Ki 11:9-13.

It is not sufficient simply to gain knowledge. It is also essential for God’s servants to act upon that knowledge, to apply it according to the divine will, yes, to do “just so".
This is a lesson that Solomon evidently forgot.


Source: Insight on the Scriptures, Volume II Pages 987, 998 Watchtower Bible and Tract Society.