I'm trying to imagine what my reaction would be like if a close friend or family member said something along the lines of:
"Kim, I've known you a long time and I want you to know I love you and I'll never stop being your friend/mom/dad/sister/whatever. I disagree with your relationship with Greg, and I can't say that what you guys are doing is okay. I know it's not what God wants. But I still love you."
I think the "love the sinner hate the sin" attitude can be beneficial in a lot of circumstances. Substitute "relationship with Greg" with "drinking problem" or "drug problem" and I could see how that could be helpful in changing behavior. I don't agree with saying "you do your thing and I'll do mine" if someone you love is truly hurting themselves (as would be the case with a drinking or drug problem).
But if someone I loved "lovingly" told me that my relationship with the person who is the most important person in the world to me was a "sin," I can't say I would take too kindly to that. Love the sinner hate the sin isn't always patronizing. It definitely has its place. None of us are perfect, and even the people we love are going to hurt us sometimes.
If people in my life insisted my relationship with my husband was a sin, I would hope I would be graceful and loving and understanding towards their point of view. But in reality, I would probably tell them to fuck off.