Yes, I can imagine it being annoying to not be believed like that.

But from a purely logical standpoint, her decision makes perfect sense. If she wants to be certain that she doesn't employ someone who ends up taking maternity leave, hiring a man or a post-menopausal woman makes that a certainty.

I'm not saying it's a kind, fair or ethical way for an employer to behave. Just that this is the logic that explains it.