Many things in this world were not independent of Neonidas' former world. After all, the person who had essentially created or at least imagined this world was Neonidas' ex-girlfriend. The fact that men held more power in this world despite both men and women having the potential to become incredibly strong mages stemmed from this reality.
In other words, the traditions—or at least the old traditions—from Neonidas' past life existed in this world. Therefore, a woman's marriage could be viewed as a kind of transaction. Especially if it was between two dukes, it would imply mutual benefits. Of course, in this case, it was not a marriage but the purchase of a concubine. However, this did not mean that interests were not prioritized.
"As I mentioned before, I wish to establish better relations, perhaps even a form of alliance, with your duchy. Thus, Duke Neonidas, I would like to strengthen our ties through my daughter, Cassandra."