In James 4:17: “Anyone, then, who knows the good he ought to do and doesn’t do it, sins.” This is a powerful scripture when it comes to marriage. Earlier I had you write out “If I was your lover I would...” You already know the good to do toward your spouse. Your heart already has a clear roadmap to love your spouse. You not doing it is sin. Not just sin against your spouse but sin against your Father-in-law, God.
How we treat our spouse is a big issue to God. So, sin is different than abuse, addiction, anorexia, and family of origin issues. These you identify or work your way through to become an awesome lover-spouse.
With sin you have to acknowledge to God and your spouse and repent. When you repent you verbally ask forgiveness and make a plan to change your behavior. Having accountability with a same gender person can keep you from going back to your sin.