The law also commands how one is to exact eye for an eye, tooth for tooth. Yet Jesus tells us not to. Same for divorce and having more than one wife. So how is using what the law allows, but which Jesus then says not to do (and explains why), a valid argument to defend doing those things?
If you were correct and that it really is now okay to marry more than one woman, I would not be committing adultery if I married another woman after a divorce. But as it is, Jesus said I would be. And so would she be, if she remarried after a divorce.
“Whoever divorces his wife and marries another woman commits adultery against her; 12 and if she herself divorces her husband and marries another man, she is committing adultery.†(Mark 10:11-12 NASB)
You can see he's plainly making the distinction between the adultery of the man for taking another wife, and the adultery of the woman for taking another husband--he's not, as you suggest, saying the woman's adultery is imputed to the man because he divorced her and caused her to commit adultery.
If it was okay to have more than one wife, Jesus would not have said the man who marries another woman after a divorce commits adultery himself by doing that. Him marrying another wife, and being guilty of adultery for doing it, has nothing to do with what the ex-wife does.