Your question doesn’t make sense. We could also say, “How can people say heterosexuality doesn’t hurt anyone, when it does damage to society by spreading disease?” To which someone might respond, “You’re right, heterosexuality can hurt people and do damage to society by spreading disease. Therefore heterosexuality is a bad thing”.
But of course no one would find that response convincing and believe that heterosexuality is bad even though it can hurt people and damage society by spreading disease, especially if heterosexual sex is practiced promiscuously or safe sex practices are not used. But it’s not the heterosexuality that is bad but the promiscuity or failing to practice safe sex that is bad.