If someone marries a girl, and then says he found she was not a virgin, whom do you believe? [She says she was a virgin.] That is easy. The Gemara in Ketuboth says on page 9b that you believe him since, "No one makes a wedding celebration just to ruin it the next day." But the Gemara says that is only because the Ketuba is not from the Torah, but rather a decree of the sages. ["They require a ketuba and they believe him if he says he did not find her a virgin."]
The Ramban [Nahmanides] asks: So if it was from the Torah, why would she be believed? Anyone coming to court with a document that has a condition in it has to prove that the condition was fulfilled.
Rav Shach explains the question and answer of the Ramban based on the gemara in Ketuboth page 76. Two people exchange animals, It comes out that you have two "hazakot"חזקות [ conditions or state of things]. One "hekat hashta" חזקת השתא [the way things are now you infer that that is the way they were in the past--and extend it as fr as possible] and the other hazaka חזקה from the start. [That is the way things were in the past you assume they keep on going until you know otherwise] If they work against each other, then you need some other kind of proof. [You see this also in the beginning of Nida].
So the Ramban answers that she has a hazaka חזקה and also a majority רוב. That is why she would be believed.
[The Gemara that Rav Shach brings says: Two people exchanged animal A with animal B. The owner of A took possession of B. But when the owner of B went to take A he found it dead. Rav Yehuda said the owner of A has to prove that A was alive at the time of the deal. Rami said no. The owner of B has to show it had died before.]
So now we understand the Ramban. If the Ketuba would be from the Torah, then you would believe her that she was a virgin because the is no Hezkat hashta [status now]against her original "hazaka meikara" [original status] . Plus a "rov" majority (Most girls get married as virgins). But without that, you would need her to show that she fulfilled the condition of the marriage --that she was a virgin when she got married. That would be like Rav Yehuda in Ketuboth 76.
So in simple English, the cases with exchange of two animals and the case of the marriage are different. In the case of the animal case, there is חזקת השתא [present status] that works against the חזקה מעיקרא [prior status]. Animal A is not alive so we push that back in time. So to Rav Yehuda, we ask the owner of animal A to show that the animal was alive at the time of the deal. But in the case of the marriage, there is no חזקת השתא [present status], since she is now anyway not a virgin. So all there is is חזקה מעיקרא --and that is exactly what the Ramban [Nahmanides] says: since there is a חזקה מעיקרא [prior status] we would believe her.
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