2 John 1:5-6 KJV
5 And now I beseech thee, lady, not as though I wrote a new commandment unto thee, but that which we had from the beginning, that we love one another.
6 And this is love, that we walk after his commandments. This is the commandment, That, as ye have heard from the beginning, ye should walk in it.

Commandment of love can and will be confused with word lust. Meaning those following lust is following sin and not love though it may feel like love. Is that confusing. If lust is why you got married your marriage may not work out. If you married out of love and lust enters in for another that not only adultry but sin despite one's emotion says your in love with another. The death in love is lust. Trying to follow read the Continual Scripture Study. When John writes in above verses his mind is on the purity of love. His teaching on love and loving one another. He is writing the church. Paul writes his church in Rome and he goes into language of finding sin in the the commandment and understanding it is sin. The commandment is tge purity of love. Two writers writing different churches and these churches being from different backgrounds. Culture of Rome love being loose term of one person loving perhaps not with one spouse or following community laws and not necessarily religious law.

Why would lust be and issue to the church that he is writing. Why is the law sinful because love in it purest form is not sin and is not confused with idolatry, or immorality. Religious laws were seldom inforces by rulers even in Bible days. The religious law understood by converts that knew the law of love belong in the puriest form without idolatry, lust and immorality. John group possibly Jewish converts need only reminded of the impure state of God's law of do not covet. Where Paul had to deal with Rome's teaching and the impurities of Gentile converts to Christians. Depending on background whether you believed in monogamist life style how scripture was applied in teaching though both taught against immorality as the heathens, followers of Roman and cultural laws disagreed with religious law. Old saying when in Roman do as the Romans. Yet why would you if both you and your spouse believe in the purity of love you dont have to live by the sin of the flesh. If you lived in marriage and in Roman and your spouse not a Christian they may not believe in the sanctity of marriage, somethings did not affect the soul only the body/flesh. Purity of faith is without immorality of sin in the flesh. Yet your situation did not change the status of your faith in God. Remember Naaman went into a temple of another god with his master. Yet he bowed but worshipped not. Loving God with your heart did not change your present circumstances but hopefully what you learn is faith is about your spiritual life and mental mindset and not your flesh. What boils down to is learning that the flesh is dead but the Spirit will live with Christ. 0ne no longer sins willfully but by action of force or inability to disobey a spouse or culture.

The purity of love being without immorality is what was taught. Purity of marriage and purity of faith with one's thoughts and actions. Though Paul teaches against immorality he knows not all will live by it so he teaches about spirituality and allowing the Spirit of God to reign in one's heart shaping one's faith first inwardly and then completely outwardly.

Dear Heavenly Father help us to love as Christ loved with a pure love not with lust or desires of our flesh. May those stuck in immoral love, find hope in the fact the flesh is compromises and not the Spirit and relationship with God. In Jesus precious name, Amen.

Continual Scripture Study: Romans 7:7-8 KJV
7 What shall we say then? Is the law sin? God forbid. Nay, I had not known sin, but by the law: for I had not known lust, except the law had said, Thou shalt not covet.
8 But sin, taking occasion by the commandment, wrought in me all manner of concupiscence. For without the law sin was dead.