When I was in medical school (1995-1999) at Northwestern University Medical School, we were taught that in DSM-3 (I believe), homosexuality was a psychiatric illness. They also discussed the possibility of it being partially a genetic illness (which as far as I know has not been confirmed). They also discussed the influence of environmental/behavioral influences (ie., upbringing, family, school, culture, society) and felt that this contributed substantially to the "condition" of homosexuality.
I believe that it is both a psychological and spiritual illness, although I have seen many cases where I wouldn't even call it an illness as it appears to be a CHOICE.I don't think that it is genetic, other than in the rare genetic illnesses I was taught about in medical school in which a baby is born with ambiguous genitalia and a choice has to be made as to which sex/gender the baby will be or where a baby is born with both male and female genitalia and again a choice has to be made and appropriate surgeries performed. These are very rare conditions!
There is a large gay community in Chicago on the north side about 3 blocks from where my condo is. It is "cool" to be gay. There is a not very subtle pressure to be gay, to act gay, to talk gay, to dress gay. If you are non-gay and walking down the streets in their neighborhood, you will be ignored or basically be invisible to them. This gay population is 95% male I believe as I rarely see lesbians and have never experienced the things I am talking about here in relation to the lesbians. In fact, honestly, I can't for the life of me pick out a lesbian on the street or in a restaurant, but I can certainly pick out a gay man by his style of dress, how he talks, what he talks about, his gestures, etc. And, I have met men who have clearly made this choice as they were previously married, have children, or are/were bisexual and have CHOSEN to be gay. It's the old nature (genetics/illness) vs. nurture (environment, family, society) argument. I honestly don't think 99% of gay men were born gay - I think they either became gay due to other issues/problems/influences in their life and upbringing or they CHOSE to become gay.
I know what I am saying is politically incorrect, but that doesn't matter because it is the truth as far as I can ascertain at this point in time. And, I still do get and read medical journals and go to CME's (continuing medical education) lectures/seminars (although they won't touch this subject with a 10 foot pole at this time in our culture/society). I intend no disrespect to any gay person and, in fact, have gay aquaintances (who I'm hoping are living a chaste life.) But, this is the best knowlege I have as a physician who is retired, but still attempting to stay up-to-date in terms of new developments in medical knowlege.
I wouldn't expect the Church to make its way into the medical aspects of the subject (and it doesn't, when the Catechism states that "(Homosexuality's) psychological genesis remains largely unexplained"), but the Church leaves no room for doubt with the line that the behavior is intrinsically disordered.