Ketubot 73A

לא תימא טעמיה דרב כיון שכנסה סתם אחולי אחליה לתנאיה, אלא טעמא דרב: לפי שאין אדם עושה בעילתו בעילת זנות.

Do not say that Rav’s reason for requiring a bill of divorce is that since he married her without specification, this indicates that he waived his condition entirely, and therefore he must give her the payment of her marriage contract if he divorces her. Rather, Rav’s reason is because a person does not intentionally engage in licentious sex. He is aware that the initial betrothal may possibly be nullified, rendering sex licentious. Therefore, when he marries her, he does so with the intention that the consummation of the marriage serves as unconditional betrothal. However, as he does not entirely waive his condition, if it becomes clear that the condition was not fulfilled, she may be divorced without receiving payment of her marriage contract.

הא פליגי בה חדא זימנא, דאתמר: קטנה שלא מיאנה, והגדילה, עמדה ונישאת ⁠— רב אמר: אין צריכה גט משני.

The Talmud asks: But they already disagree about this fundamental issue of whether it may be assumed that a person does not intentionally engage in licentious sex one other time. As it is stated: With regard to a minor girl whose mother or brother married her off, and who did not refuse her husband, despite having the right to do so, and when she became an adult she arose and married someone else, Rav said: She does not require a bill of divorce from the second one. Since she did not refuse the first husband while still a minor, and presumably he later had sex with her when she became an adult, and since the assumption is that he does not intentionally engage in licentious sex, the first marriage is binding and the second is meaningless.