We se here the first example of mankind going against God’s rules on marriage by taking two wives. Nowhere does God say it’s okay to have more than one wife. He allowed in in his love and patience toward people at times, but it was always sin, going away from His intended purpose.
Note, too, that Lamech took the wives. Nothing is sais about God marrying them, or even endorsing this action. By this time, mankind – even Seth’s line – had begun to pollute the world with wickedness, which would eventually lead to the destruction of all flesh.
Notice, too, that this was the type of thing God warned Even would happen. The stronger man, accustomed to hunting and other physical labor all his life, felt that he could force his wishes upon women. As we’ll see in verse 23, he was that way with everyone, so it makes sense that he would think he could just take whatever he wanted through brute strength, not considering others and certainly not considering God.