Some "criminals" can be taught to "think differently" or "retrained", others we simply do not have the technical ability to repair AT THIS TIME, but the primary question should be "can we modify the behavior or not? "
If we can not, the only available option AT THIS TIME is to remove the individual from society, but if we can, we should correct the dysfunction and then return the individual to society. Punishment is utterly useless in this paradigm, and any educated person recognizes this.
Getting back to your statement; another inaccuracy is the premise that "Only within the last few decades, however, has the lifestyle been de-criminalized". In THIS Country (we are the "west" that you are speaking of, aren't we?) Indian cultures often revered homosexuals, several of the "European cultures" that form the bulk of our population accepted homosexual behavior long before they arrived on these shores, and many other cultures have both celebrated those who were different or simply accepted that they were an oddity that posed no threat to the culture as a whole. ONLY HERE, in those parts of the country that latched onto the fundamentalist types of Christianity that sees the world in terms of absolute right and absolute wrong (black and white), where they could read in the bible that their personal bias and xenophobia and prejudice were justified (by selective reading) has the utter condemnation of an entire class of people been justified.
About Gay Marriage
Read the Article at HuffingtonPost