If I could just respond, I respectfully would disagree with my colleague. I believe that for some individuals it may be a mental disorder; for others, really, it's not and it's an issue of power and control.
I don't currently work with sex offenders, but I have in my career. The ones who are able to benefit from treatment are the ones who are also willing to admit and take responsibility, and in my experience that's not the majority of sex offenders. I believe that for many, prison is necessary in order to have some leverage to get them into treatment. And there are many good institutional treatment programs that can be initiated while somebody is in custody and they can continue treatment in the community. But for many sex offenders, taking part in treatment is not necessarily something they agree to very easily.