I could give some responses to how homosexuality can be destructive, and they would be centered around ideas completely foreign to your beliefs such as what sexuality was actually created and designed for by God, the covenantal representation of Christ and his bride, etc., and others that you might understand but still disagree with such as the healthiness of a mother and father in raising a child and the history of that being the sanctuary for bringing forth and bringing up children, but ultimately those would be falling on deaf ears because you don't hold Scripture in high regard and agree. Which I'm fine with, but I'm also ending the conversation at "I disagree with you", and letting you keep on thinking what you want.
But even then, reducing immorality to the basis of whether or not it causes harm is a bad idea. What if a man cheats on his wife but she never finds out because he hides it well? No harm comes but are we going to say that isn't wrong? There are tons of things that people consider wrong that aren't immediately and demonstrably harmful - don't act like you don't believe in some things being wrong that aren't obviously harmful.
Imagine typical reactions to someone burning an American flag, or what your response would be if your dog was killed by a car on the highway and I suggested we eat the meat. All of our moral codes are filled with tons of things that are perceived wrong by more than whether or not they're harmful.
You could give those responses and I would point to studies that show that children from same sex couples are no worse off than those from heterosexual couples. It's not that it's falling on deaf ears, I'm genuinely interested in the debate and glad that you're more progressive than the majority of Christians in this country, I just don't understand how you can compare a person's genuine love for another human being to alcoholism and drug use. I guess that's where we disagree.