Romans 15:1Our society seems to think that in order to love someone we must fully agree with them and support their actions no matter what. God is not satisfied with that mentality in our lives, always lovingly correcting us and guiding us to repent of our sins. We should imitate the Lord rather than the world in the way we choose to relate to others. We love them, but that doesn't mean we agree with everything they do. We give them leeway to reform their sinful tendencies and flexibility to be their own person, yet we don't encourage the person we love to openly sin.
Since we take the Lord for our role model, how does He treat us when we are sinning? He always relates to us with love. He doesn't look the other way when we sin. He allows us plenty of flexibility to conquer each type of sin: making the process of sanctification a lifelong process rather than an overnight project. Yet as soon as we reach a measure of victory over one area of our lives He calls us on to begin work on another area, so that we may become holy as He is holy (1 Pt 1:16).
So, when someone we know is sinning in a serious manner, we love the person first and foremost but we don't pretend that the sin is okay. We encourage one another in efforts to overcome our sinful tendencies. Challenging one another to avoid sin doesn't show a lack of love.