Homosexuality has been shown to be caused by three things, Molestation, insufficient or absent relationship with one's father or severe teasing/rejection by one's peers during the formative years. (Usually two or all three causes are present)
The first cause results in damage to the human mind and spirit. This damage results in that person choosing to re-commit the behavior of the molestation in an attempt (sub concious) to identify with the attacker. (If he did it to me than maybe it's not my fault and it's just normal. If I can be just like him than I don't have to feel this shame of being violated.) This occurs in the female form of the damage also. My attacker likes girls so I'm supposed to like girls also. The basic human condition has been warped and twisted.
The second cause forces the person to seek to replace that missing father's love, trying to fill the hole that's left behind. But like the hole that is reserved in us for Jesus, only the authentic thing will fill the need, so the person continues to seek for that manly love that will fill his emotional need. He is damaged emotionally. (This shows up as promiscuity in girls deprived of their father's love also)
The third cause becomes a self-fulfilling prophecy. (If everyone says I am than maybe I really am)
All of these result in a behavior that is sinful. But I've found that to break the habit of the sin you have to address the underlying need. Just as to break most addictions you have to meet the need that people are searching to fill (usually Jesus).
So, while the person practicing homosexual behavior is totally responsible for that sinful behavior, they are also damaged and broken people. Just calling them sinful without giving them a way out doesn't really help them much. It tends to push them into the 'God made me this way' talking point. Telling them they are sinning and giving them a way out is more effective. Both secular (NARTH) and religious (Exodus) therapies have been proven to be successful for those who want to be free.
Of course sometimes God does deliver someone totally when they get saved. At other times He chooses not to. I have known people in both conditions. I have also known people who were practicing Christians, on fire for the Lord, leading worship and raising Godly children, who never fixed the causes described above and as a result of that damage, fell into a habit of sin.
God gives us understanding into how He designed us so that we can heal each other. Just as a medical doctor has been given understanding of how to heal the body the mental and/or spiritual 'doctor' has a responsibility to heal the mind/spirit.
So, call it mental illness or call it habitual sin, the person practicing homosexual behavior needs help. They are twisted and damaged and must be restored.
I call homosexuality a disease because it is both curable and contagious. People are recruited into the practice of homosexual behavior (as shown by the 'gays' insistence that this practice be taught in the schools).