The abnormality of it is self-evident in that their sex organs do not match up and their biological design is entirely heterosexual. Lesbians ovulate. Whatever for? etc.
Still I hold that the underlying issue is sin. One sin leads to another leads to another. Something traumatic may have set them on this path, but the guilt of the sin, both the desire and the deed, is on them. Same goes for all of us. People sin against us; that gives us no excuse. We are still 100% guilty before God.
Immorality is always a vice. It might also be abnormal, but it is still a vice. It's an odd place to be self-righteous, but too many take that approach. To refuse responsibility is self-righteousness. Someone or something else is to blame. Child abuse is a horrible offense and I mourn deeply for the sufferings of those who endure it. But even that is no excuse.