Is the argument "people will do it anyway" a bad one for making something legal?

I hear this argument made all the time in reference to abortion and drugs. Is it a bad argument? People will kill people anyway; that doesn’t mean murder should be legal. But does it have some validity? Prohibition was a failure; people “drank anyway” and it caused new problems. Thoughts?

Please no one-word “yes” or “no” answers. Explaining your reasoning! :)