But it is not the therapeutic aspect that would be immoral. The context on that thread was "designer babies", and you yourself seem to suggest it by making the contact lens analogy.
Beside destruction of embryo, even in therapeutic use we should distinguish between a health defect and cosmetic improvements. Further, the Church doesn't like any usurpation of the power of God as creator of life.
The discussion covered both cosmetic changes (like eye color) and health defects (such as handicaps and vision correction). I did not see any distinction drawn between the two during that discussion.
Beside destruction of embryo, even in therapeutic use we should distinguish between a health defect and cosmetic improvements.
What would be the problem with making a genetic change, for cosmetic purposes, to a naturally conceived zygote? Is it the child's lack of choice that makes it immoral, or is it the change itself that is morally problematic?
If it's the cosmetic change itself that is immoral, does this also mean it would be immoral for me to shave every morning and afternoon of my adult life? Or for me to get some kind of advanced permanent laser hair removal? (Assuming that it was for purely cosmetic reasons and there was no health necessity for being clean shaven).