There can be reasons for obeying God's law other than trying to earn our justification, especially because God's law was never given as a means of doing that, so verses that speak against earning our justification by our obedience do it should not be mistaken as speaking against our justification requiring our obedience to it for some other reason, such as faith. In Matthew 23:23, Jesus said that faith is one of the weightier matters of the law, so only those who have faith will obey it and will be justified by the same faith, which is why Paul could say both that only doers of the law will be justified and use Abraham's example to show that our justification is not earned as a wage. While it is true that Abraham believed God, so he was justified, it is also true that he believed God, so he obeyed God's command to offer Isaac, so the same faith by which he was justified was also expressed as obedience to God, but he did not earn his justification by his obedience as a wage. In James 2:21-23, Abraham was justified by his works when he offered Isaac, his faith was active along with his works, and his faith completed his works, so he was justified by his works insofar as they were an expression of his faith, but he was not justified by his works insofar as they were earning a wage.
So, yes, I would tell a teenager that thinking about a woman with lust in our hears is breaking both the 7th and 10th Commandments against adultery and against coveting in our hearts and that the same faith by which we are justified is expressed through obeying the Ten Commandments as well as God's other commandments. I agree that in Romans 2:13 that Paul was not speaking about earning our justification as wage and that there are many verses that speak against earning our justification, though it is nevertheless still true that he was saying that only doers of the law will be justified.
Paul spoke about multiple different categories of law other than the Law of God, such as works of the law and the law of sin, so it is important to correctly identify which law he was speaking about us not being under. For example, in Romans 3:27, Paul contrasted a law of works with a law of faith, and in Romans 7:25, he contrasted the Law of God with the law of sin. If Romans 7:22-23, Paul said that he delighted in obeying the Law of God, but contrasted that with the law of sin, which held him captive, so if 7:5-6 were speaking about the Law of God, then that would mean that Paul delighted in stirring up sinful passions in order to bear fruit unto death and that he delighted in being held captive, which is absurd, but rather it is the law of sin that he described as holding him captive. So throughout Romans 7:7-13, Paul is also going back and forth speaking about the Law of God and the law of sin. In verse 8, it is the law of sin sizing the opportunity through the Law of God to produce all kinds of covetousness, so it is apart from the law sin that sin lies dead, which again means that it is the law of sin that is the power of sin in 1 Corinthians 15:56. In Romans 7:12-13, the Law of God is holy, righteous, and good, but it was not what was good that brought death to him, but rather it was the law of sin that was working death in him. So yes, the law against coveting is part of the Law of God, but it is the law of sin that is the power of sin, not the law of God, and a law that is holy, righteous, and good can't be the power of sin because sin is the opposite of what is holy, righteous, and good.
In regard to Romans 6:14, Paul described the law that we are not under as being a law where sin had dominion over us, which does not describe the Law of God, which is a law where holiness, righteousness, and goodness have dominion over us, but rather it is the law of sin that Paul described as sin having dominion over us to cause us not to do the good of obeying the Law of God that we want to do. In Romans 6:15, being under grace does not mean that we are permitted to sin, and sin is the transgression of the Law of God (1 John 3:4), so we are still under the Law of God. Furthermore, everything else in Romans 6 speaks in favor of obedience to the law of God and against sin. For example, in Romans 6:19-22, we are no longer to present ourselves as slaves to impurity, lawlessness, and sin, but are now to present ourselves as slaves to God and to righteousness leading to sanctification, and the goal of sanctification is eternal life in Christ, which is the gift of God, so being under God's law is part of the gift of God.
In Matthew 19:17, Christ said that the way to enter eternal life is by obeying God's commandments, so again, that is part of the gift of God. In Psalms 119:29, David wanted God to be gracious to him by teaching him to obey His law, so that is the gift of God. In Titus 2:11-14, our salvation is described as being trained by grace to do what is godly, righteous, and good, and to renounce doing what is ungodly, so again God graciously teaching us to obey His laws for how to do that is part of the content of His gift of salvation.