I first noticed that discrepancy years ago. Knowing that God does nothing by accident, I have prayed and researched to find the answer. I have a theory, but that is all that it is.
Absolutely.
Yes. God and the human scripture writers were not shy in saying EXACTLY what was not allowed. So any omission
had to be intentional.
My theory is based on God's mercy and love for us, His creation.
Another interesting omission is God including scriptures such as this:
Exodus 21:10
If he takes to himself another woman, he may not reduce her food, her clothing, or
her conjugal rights.
God says that even in a multiple wife marriage, the "conjugal rights" cannot be diminished. IOW, God is concerned that
she has regular/frequent sexual satisfaction. No such command exists concerning husbands' sexual satisfaction. And I believe that was intentional as well.
Now to my theory: Remember Solomon who had 700 wives and 300 concubines? That is 1000 women he is (by the law of Moses) responsible to keep sexually satisfied. Do the math. If he has 3 women each night, it would take almost a year to get to each one only once.
If you are a wife, would that be often enough?
NO.
And could he even keep up such a pace for that long? VERY Doubtful.
So God allowed the women to legally "take care" of each other. While it was not the best idea, or ideal in any way, at least there was a legal way to reach satisfaction.
With the demise of complex marriage in the late 2nd temple period, God finally had Paul describe that situation as being a fruit of the fallen nature of mankind.
Please remember, that is NOT scripture. It is only my theory.
Remember, the bulk of scripture (both OT and NT) is mitigating the ill effects of fallen humans living in a fallen world. There are things allowed in scripture (like divorce) that God dislikes, but allows for our sake and safety.