Actually, you’re over-generalizing and over-simplifying.
God ‘allows’ many things in that if, for example, somebody shot a bullet and killed you (a mortal sin), God Himself wouldn’t be causing you to be killed (that would be on the person who murdered you) but allowing that person’s free will (in killing you) to occur. Since physics postulate that a bullet’s trajectory into a person’s body if said bullet hits a vital organ and causes that failure, resulting in death, God is ‘allowing’ the action of the law of physics to pretty much ‘follow the law’.
So yes, in the OT there were quite a few cultural practices that God allowed to exist based on the ‘free will’ of persons. He didn’t just decide that “OK, here’s David, married once, now he’s marrying somebody else, the law of the universe is if that happens, he goes up in smoke” because that isn’t what normally happens if a person goes through two, three, or more marriage ceremonies. It’s not the same as physics.
BUT God did tell the people even then what marriage SHOULD be. Because people were ‘hard of heart’, He even allowed them to have secular laws that permitted two wives, or divorce, because the ‘real law’ was there, and He hoped people would come ‘on their own’ to see how badly their ‘secular laws’ worked out. And they did work out badly.
And when Jesus came to fulfill the law He simply told them what God had already said in the OT, and why He had ‘allowed’ difference, and how badly it had worked, and what they SHOULD do (which is what God had said all along, one man/one woman, till death do they part).
Thing is, people are still ignoring him.
God’s morality didn’t change from OT to NT, and things weren’t OK with Him in one century and ‘not OK’ in another.