I think marriage is a monogamous institution by divine definition. However, not all relationships are marriages and therefore not all are monogamous. Marriage is a spiritual choice in obedience to God, not a function of “nature; “nature” allows humans to be single as well or to design any number of alternate structures.
Marriage is a choice made by a man and a woman who have already made a choice for God and that institution must be monogamous.
You must be prepared by faith, spirit, and temperament by God to undertake the responsibilities of a husband or a wife. Many are not ready, by virtue of lacking in these areas, for the requirements of these sacred obligations and therefore seek or end up in other arrangements.