"Asexual" people are eunuchs.
St. Thomas Aquinas writes in his Commentary on St. Matthew (19:12):
For there are eunuchs, etc. Hence, He distinguishes the continence that is in some men by nature…because certain men are eunuchs by nature, who were born so from their mothers’ womb.
This first type of eunuch needn't refer only to those born without genitals but also to those lacking sexual desire. St. Thomas also mentions that people have various degrees of frigidity/calidity (cf. Summa suppl. q. 58).
"Asexuality" is not synonymous with chastity/virginity.
Discussing whether virginity is a virtue, St. Thomas Aquinas poses the following objection (Summa Theologica II-II q. 152 a. 1 arg. 1):
Objection 1: It would seem that virginity is not a virtue. For "no virtue is in us by nature," as the Philosopher says (Ethic. ii, 1). Now virginity is in us by nature, since all are virgins when born. Therefore virginity is not a virtue.
to which he responds (ad 1):
Reply to Objection 1: Men have from their birth that which is material in virginity, namely integrity of the flesh and freedom from venereal experience. But they have not that which is formal in virginity, namely the purpose of safeguarding this integrity for God's sake, which purpose gives virginity its character of virtue. Hence Augustine says (De Virgin. xi): "Nor do we praise virgins for being virgins, but, because their virginity is consecrated to God by holy continency."
Similarly, some people are born with a genetic predisposition to alcoholism, for example, but that does not mean those without this predisposition necessarily practice the virtue of sobriety.