A friend of mine once told me that he thought that if there were laws passed against abortion, they would never be obeyed and never be enforced. Forget about the back alley abortionists. The front door of the abortion clinics would be open, and law enforcement would be powerless to enforce the law.
Now if a law is unenforceable, perhaps we are morally obligated to have the law anyway. But it would be worthless as a deterrent.