Arabella learned that Ferdinand was particular about it since he had been overhearing the knights and other people around him talking about those in low voices.
He heard that there were those who were left by their lovers or divorced by their wives because their thing down there could not function properly; most especially if they could not get a woman pregnant.
Ferdinand kept hearing people talk that a man's value in their society now was also judged by whether they could sire a child or not.
Thus, he also ended up thinking he could be laughed at, divorced, or abandoned due to it. And he was more on edge now after Arabella told him about Eliana and he recalled that he was already left behind before because of such an issue.
He thought Eliana left him because they could not have children due to his body not working as it should have been.