A Legitimate Marital Act,not a Promiscuous One

It was taught: If a person betrothed a woman on a condition (that she had no current vows), and he later married her without mentioning that condition (and she was later found to be in violation of that condition), there is an argument between Rav and Shmuel. Rav says that she requires a divorce, while Shmuel argues that she does not require a divorce.

Abaye states: Do not say that the reasoning of Rav is that because he married her without mentioning the condition he must have forgone the stipulation. Rather, Rav’s reasoning is that a person does not make his marital relations into promiscuity.

The Rishonim ask: The cohabitation in this case is anyways a promiscuous, non-marital cohabitation! This is because we learned previously (54b) that if anyone reduces the prescribed kesuvah amount from his wife, any acts of cohabitation is regarded as promiscuous. In our case, she does forfeit her kesuvah since the husband stipulated that she should not be under any vows, and since it emerged that she is under a vow, he would not be obligated to pay her kesuvah! Accordingly, what advantage is there that he does not want his cohabitation to be a promiscuous one (and therefore, the betrothal is valid), it is anyways regarded as a promiscuous one?

The Ran answers: A person is particular that he will not intentionally render his cohabitation to be regarded as a promiscuous one. However, he is not particular if the Rabbis render his cohabitation to be promiscuous (and it is the Rabbis who deemed it to e a promiscuous cohabitation, when he cohabits with a wife without a kesuvah).