Why is loving someone a sin?

I’m sure this question has been asked here many times. This is about lgbtq+ being a sin.

I was reading Mere Christianity by C. S. Lewis and he was saying that there’s this moral law that humans should follow but we don’t (and we usually know when we’re done something wrong) and that the law is evidence of God’s existence. Guiding us how to behave. Which makes sense. It makes sense that we shouldn’t harm ourselves or anyone else. Isn’t that what Christians would call sin? What I have never understood is why love would be a sin. Sins like killing, stealing, gossiping makes sense but being in a gay relationship doesn’t harm anyone. Instead the fruit is love. When I commit any other sin, I feel convicted. I know I’m doing something I shouldn’t do. But when I show my support for the lgbtq+ community, I’m not convicted.