Mishnah 3: In the previous mishnah we learned that a girl under the age of three, even if she was cohabited with, has no halachic status attached to that act at all, and she remains a virgin as before. The same is true of a boy under the age of nine: any act of cohabitation he performs is not considered a real act, and it does not remove the woman with whom he cohabited from the status of a virgin.
On this basis, our mishnah deals with three cases:
An older boy who cohabited with a young girl - a boy over the age of nine who cohabited with a girl under the age of three.
A young boy who cohabited with an older girl - a boy under the age of nine who performed any act of cohabitation with a girl over the age of three.
Mukas eitz - a girl with whom no one cohabited at all, but to whom it happened that a piece of wood or some other physical penetration struck her and broke her hymen, so that physically she is no longer a virgin, but not through marital relations.
The opinion of Rabbi Meir:
According to Rabbi Meir, each of these girls is still considered a virgin with regard to the law of the kesubah, and none of this diminishes her kesubah at all. In the first two cases there was no act of cohabitation at all, and her virginity is intact; and in the case of a mukas eitz, even though technically she has no hymen, since this did not come about through an act of cohabitation, her kesubah is two hundred.
The opinion of the Sages:
They disagree with Rabbi Meir in the last case only, and say: "Mukas eitz kesubasah maneh" - the kesubah of a mukas eitz is a maneh. In their view, the very fact that physically she no longer has her hymen, even though this did not come about through relations with a man, is enough to fix her kesubah at a maneh alone. This is not comparable to the first two cases, in which she is a full virgin, since the relations that took place are not considered an act of cohabitation at all in halachah.
In summary: In this mishnah we learned of three cases in which, according to Rabbi Meir, the status of a virgin is not removed with regard to the kesubah - an older boy who cohabited with a young girl, a young boy who cohabited with an older girl, and a mukas eitz - and in all of them her kesubah is two hundred. The Sages disagree in the case of a mukas eitz alone and hold that her kesubah is a maneh, because in practice she has no hymen, even though this did not come about through an act of cohabitation.