I'll answer you out of the theological doctrine I believe to be true. This is not advocacy for laws, just what I believe to be true, and how it applies to the spiritual and emotional nature of mankind.
He didn't design it to work that way. We are designed to join one man to one woman, with God overhead. When we insert more people into that equation, we throw it off, and risk emotional and spiritual harm to ourselves and others.
Think of it like a piece of play ground equipment. Misusing it may be fun, and may not even seem bad if nobody is getting hurt. But if that misuse breaks the equipment, you don't get to have fun anymore.
Not sure I truly agree here... I mean, lots of guys in the OT had plenty of wives, and with God's blessings. Culturally, we've evolved to a more one-on-one acceptable style, but I'm not sure "we are designed" that way.