Catholic doctrine maintains that homosexual orientation per se, is not sinful but disordered. Homosexual acts are acts of "great depravity" and the lust or desire to commit such acts would also be sinful.
See HERE
As I suspected, it is the act and the lust for the act that is considered sin, not merely the lack of desire toward the opposite sex.
Those kinds of persons have always existed, they lack strong sexual desire, certainly for the opposite sex.
They were always the unmarried uncle or aunt in the family.
Sometimes they were referred to as the spinster auntie or the perennial bachelor uncle.
Many times we looked forward to their visits for they oftentimes brought gifs for the kiddies as they weren't spending their money on their own families!
Now, however, society pushes them toward same sex affiliations as society cannot imagine life without sex!