Lilliny
That's pretty easy as well. A virgin has never had anyone else. She is his alone. He gets to conquer her utterly, and he will by default be the best sex she's ever had. Additionally, his reproductive lineage is secure if he has been her only partner when she is impregnated, thus ensuring his bloodline continues.
The entire concept of virginity harks from Bronze-age social and legal systems that treated women as property that served as baby factories for men, and gave the father's entire estate and inheritance to the first-born male. If a woman was not a virgin, the idea was that she was impure and the father would be unable to guarantee his woman's offspring would be his.
This owning of a woman's chastity for patriarchal reasons has been around for thousands of years, and still manifests in rituals such as purity rings, purity balls, virginity contracts, hymen examinations, and other such nonsense to this day. There is no onus on men to be virgins because they are autonomous agents who cannot be impregnated. They are people. For most of history, women were not. And we're still seeing the widespread continuation of backwards and problematic ideas and behaviors as a result.