Is monogamy Biblical?
It isn't. in Old Testament times, it was perfectly normal for a man to have both concubines and several wives. But that was no invitation to licence. There were strict rules about how multiple wives were to be treated. All wives had extensive rights. As it says in Exodus 21:10: "If he takes another wife to himself, he shall not diminish her food, her clothing, or her marital rights."
It is only in the NT that we see a move towards monogamy and there is is not any sort of commandment. It is advice. As Paul says in 1 Cor. 7 "But because of the temptation to sexual immorality, each man should have his own wife and each woman her own husband."
This made made clearer in 1 Timothy 3: "Therefore an overseer must be above reproach, the husband of one wife, sober-minded, self-controlled, respectable, hospitable, able to teach". So it was only the officers of the church to whom the advice applied and the reason for the advice was that it made the officer look good, not that it was right or wrong.
It may be argued that in Matthew 19 Jesus commanded monogamy. There are two objections to that. The first is that Jesus was very clearly on that occasion aiming only to confound the Pharisees and the second is that Jesus was actually forbidding divorce, not forbidding second marriages: "What therefore God has joined together, let not man separate.”