Q. [Pre-cana priest] Are you a Virgin?
A. [First appropriate response] May I ask why? [establishing relevancy vs. morbid curiosity]
Relevancy: In the case of pre-marital sex being the reason for the loseloss of virginity.
Reading the question, I was thinking under what circumstances pre-marital sex might come up and it is in, for example, the following two elements in Marriage Preparation Policies:
4.Moral issues (cohabitation, choosing not to have children, abortion)
and
12.Readiness issues (abuse, addictions, age, pregnancy, mental illnesses, immaturity, brief courtship, marriage on the rebound or as rebellion, lack of financial support) cf. An Analysis of Diocesan Marriage Preparation Policies.
Say pre-marital sex is present at the time of marriage but the sex is with someone else other than with the party marriage is being contracted with. That could be adultery at the time of marriage and if proven later by the innocent party when marriage later breaks, that could be a valid issue for annulment.
The probing into pre-marital sex and the underlying reasons, may also help guide parties to the proper understanding of marriage as the Church understands it.
Concluding, honest responses on all elements of the marriage preparation policy are to be made to the priest conducting pre-cana.