How is masturbation not a sin when it involves lustful thoughts or fantasies?

I've seen a lot of people in this sub try to argue that masturbation isn't a sin. And that's fair, as calling it a sin leads to a very unhealthy attitude towards sex. BUT, you only do it when you're horny, and while that's not a sin in itself, you're gonna fantasize. I guess the answer is you don't? But you're most likely gonna anyway, and then feel bad for it, which isn't healthy.