St. Joseph—renowned for his chastity—as painted by Guido Reni in 1640.
***
EDIT: Both this and the subsequent post make arguments very similar to that made here by Michael Beaty.
Suppose that we are virtue ethicists (as we should be), and we accept that chastity is a real virtue (as it is). Then the following argument opens up:
Chastity requires that one cultivate habits of constancy and fidelity for the sake of one’s future marriage.
Premarital sex and cohabitation tend to reduce constancy and fidelity.
Therefore, chastity requires that one avoid premarital sex and cohabitation.
Premise (1) seems quite plausible: if chastity involves anything, it would seem to involve making ourselves into the sort of people who are capable of flourishing, committed partnerships. (Maybe one could question whether lifelong marriage is indeed the ideal form that a flourishing sexual relationship should take. But I think there are good arguments to that effect.)
Premise (2) is justified on empirical grounds: as far as we can tell, it just is the case that premarital sex and cohabitation lead to higher rates of marital infidelity and spousal separation.1 As such, it seems that these practices run contrary to the duties imposed upon us by the virtue of chastity. The conclusion (3) follows logically.
For the link between premarital sex and infidelity, see Haseli, Shariati, Nazari, Keramat, and Emamian (2019), as well as references therein. For the link between premarital sex and divorce, see Smith and Wolfinger (2023). For a discussion of the (extensive) literature on cohabitation and marital outcomes, see Kaczor (2022).
Note that the link between premarital sex and divorce was found to be “robust to controls for beliefs and values, religious background, family relationships, personality characteristics, and mental health in adolescence” (Smith and Wolfinger 2023, 2). This would seem to contradict the claim that the link between premarital sex and divorce is purely a matter of correlation, with religious conservatives avoiding both simply out of adherence to dogma.
Also worth noting: research indicates that people with exactly one lifetime sexual partner (i.e. their spouse) have the happiest marriages. This rebuts the claim that the aforementioned data reflects people staying in unhappy marriages.