We are studying Mishnah 5 in the first chapter of Maseches Kesubos. The Mishnah opens: "Ha'ochel etzel chamiv biYehudah shelo b'edim - eino yachol lit'on ta'anas besulim" - one who eats at his father-in-law's house in Judea without witnesses cannot make a claim about virginity.
The Custom in Judea:
The Mishnah is speaking about a man who is betrothed - one who has performed kiddushin with a woman but has not yet married her - and who eats in his father-in-law's house and spends time with his betrothed. In Judea it was customary to allow the betrothed couple to be alone together, so that they would come to know each other and grow close, and as a result intimacy would come more easily to them once they married.
The Law That Follows:
When they were secluded together without witnesses, with no one seeing them, the husband can no longer claim after the marriage: "I did not find her a virgin." The reason: since he was already secluded with her, it is entirely possible that they had relations at that time, and if he does not find her a virgin afterward, he himself is the cause. He therefore has no claim at all regarding her not being a virgin.
The Kesubah of a Widow - the Daughter of an Israelite and the Daughter of a Kohen:
The Mishnah continues and discusses the different kinds of widows: "Achas almenas Yisrael v'achas almenas kohen - kesubasah maneh" - whether she is the widow of an Israelite or the widow of a kohen, her kesubah is a maneh. Whether the widow is the daughter of an Israelite or the daughter of a kohen, and even though kohanim have greater lineage, the value of every widow's kesubah is a maneh.
The need to emphasize this point arises from what follows: "Beis din shel kohanim hayu govin l'besulah arba me'os zuz, v'lo michu b'yadam chachamim" - the court of the kohanim would collect four hundred zuz for a virgin, and the Sages did not protest. The court of the kohanim collected four hundred zuz for a virgin who was the daughter of a kohen instead of the usual two hundred, and the Sages did not protest, since they saw it as fitting that a daughter of higher lineage should be able to claim a larger kesubah.
And why was the same not done for a widow?
With a widow they did not make this distinction. Had they set two hundred zuz for a widow who is the daughter of a kohen as opposed to a maneh for a widow who is the daughter of an Israelite, a man would say: why should I spend two hundred zuz on a widow who is the daughter of a kohen, when for that very same sum I could marry a virgin who is the daughter of an Israelite? Out of concern that men would prefer the virgin daughter of an Israelite over the widowed daughter of a kohen, they left the kesubah of the widowed daughter of a kohen at the same amount as that of the widowed daughter of an Israelite.
In summary: In this Mishnah we learned two matters: one who eats at his father-in-law's house in Judea without witnesses cannot make a claim about virginity, because he was secluded with her and it is possible that he himself had relations with her; and the kesubah of a widow, whether the daughter of an Israelite or the daughter of a kohen, is a maneh. Even though the court of the kohanim raised the kesubah of a virgin daughter of a kohen to four hundred zuz and the Sages did not protest, in the case of a widow they did not raise the amount, so that men would not refrain from marrying her.