Yes, I have known a woman who had a long affair with a priest and made sure she never fell pregnant. I think the argument your students use overlooks two issues:
1) in the past women were not always so savvy, and in some cultures still aren't (indeed it can be a matter of 'innocent' pride or affirmation that a priest finds them attractive).
2) a vulnerable and/or previously abused girl/woman is completely disempowered and is unable to take control of her own body and her own feelings. They submit to figures of authority, and are easily moved by apparent kindness and gentleness.
It is these women who need protection.

Where it is a mutual, knowledgeable attraction that is deliberately pursued by both parties, then you are correct - the woman must take equal responsibility for the consequences.