The Biblical doctrine of Eternal Security does not give a believer a "license for immorality." Romans 6:15 plainly teaches that the born-again believer is NO LONGER UNDER THE LAW; yet is still expected to fulfill the Law... "What then? shall we sin, because we are not under the law, but under grace? God forbid." But fulfilling the Law is a result of genuine repentance and NOT a prerequisite or requirement for salvation.
Again in Romans 3:31 we read, "Do we then make void the law through faith? God forbid: yea, we establish the law." Just because sinners are saved without works (Romans 3:20) does not give anyone permission to sin. Opponents of Eternal Security make the mistake of placing the cart before the horse, i.e., they expect people to continually forsake their sins to guarantee salvation; but it's the other way around, we depart from our sins because we are saved and love the Lord.