The Ketzot gives a novel answer. Contrary to common belief, the Ketzot maintains that witnesses required in commercial transaction are also eidei kiyum – that is to say, their testimony is required to give make the loan effective. So why is the defendant obliged to pay when he admits he owes the money, if despite the admission the loan is not effective in the absence of witnesses? The answer, explains the Ketzot, lies in the deeper meaning of “hoda’at ba’al din keme’ah eidim damei.” An admission of a defendant is so powerful that it is as if 100 witnesses actually came to court to testify.
However, the admission of a defendant is admissible if it disadvantages only the person making the admission but not if it disadvantages someone else. For example, the admission of a borrower, in the absence of witnesses, would not be accepted if the loan was guaranteed by a third party and resulted in the guarantor, rather than the borrower, having to pay.
Similarly, explains the Ketzot, the admission of the couple that they married is not accepted because it disadvantages others. Other suitors who would have been permitted to marry the “bride,” would now be barred from doing so if the court would accept the admission and render her an eshet ish, a woman who is forbidden to have relations with others.