Please explain why Ketuboth 1:6 states that she lost her virginity
before
they were betrothed.
This is the charge made by the husband in Ketuboth i 6, because he
feels he has been cheated and he basically wants his money back. The
bride's claim in this Mishnah is that she was raped after the
betrothal, which if accepted as true means the husband doesn't get to
lodge a suit against her to get his money back even if he chooses to
divorce her. The woman cannot be stoned in this example, regardless
of the facts, since the husband is claiming that she had sex with
someone prior to their engagement.