Let's say Ben is your brother and his partner Jerry want to come visit in your home. They are “married” and proclaim their gay lifestyle openly. They expect to be welcomed as family members, to stay in the guest room together and you're supposed to act as if this is perfectly normal. But let's say you have a houseful of children, and you are doing your best to bring them up to understand what a true Catholic understanding of marriage is. You've taught them that Ben and Jerry's relationship is not that. So what are you going to do?
You have to choose between two goods.
Sometimes the best way to love the sinner and hate the sin is to speak clearly to the sinner about the situation and warn him.
The good of preserving your children's understanding of Christian marriage without confusion or compromise is a greater good than being nice to Ben and Jerry. After all, Ben and Jerry are not members of your immediate family, and although you might want to be nice to them, your duty to your children comes first. This is no different than any number of other choices we make between two goods. I want to take my kids on a grand European vacation, but I also want to pay for their college education. I choose the college education.
Therefore in the wish to be nice to everyone and love the sinner while hating the sin sometimes the sinner is going to get knocked. It can't be helped.
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Finally, true love for the sinner is to see them for who they really are and to see past whatever problem, addiction, brokenness or confusion they suffer from and to wish God's perfect healing love to be active and fruitful in their lives. We're all a mess, and the sooner we realize it the better, and the person who is most compassionate is the one who realizes what God's amazing grace has done for them and how they have been rescued and to wish that same deliverance for others.
So it is first in our own conversion and the long, hard road of repentance, reconciliation and renewal that true love for the sinner and hatred of sin is fostered. Hatred of sin because we see how it has destroyed our own lives, and love for the sinner because we can see what they might be and who they could become if they were only to yield to that amazing grace.
That's why St Julian of Norwich writes that God looks on us with pity not with blame. His mercy is everlasting. He loves the sinner because he sees what that fallen, broken child could become, and he hates the sin because he sees how it has diseased and deformed the child.
When we begin to have those kind of everlasting eyes we can begin to hate the sin and love the sinner.
Everything else is either sentimental clap trap, wishful thinking or do-goodism.