Well, I’d just ignore her and go about my business.
But, if you want actual responses:
(1) The Church herself recommends it. What makes mom think she knows better than the Church? In fact, in a number of dioceses it is required as part of marriage preparation. Your mom should study up on what the Church calls “proximate preparation for marriage” and sex information is a part of that preparation.
(2) NFP needs to be studied without intercourse, a woman cannot learn her cycle as effectively if there is seminal fluid present.
The Church disagrees-- again proximate preparation for marriage includes sexual topics.
Well, the best response I can give to this is Canon Law:
Can. 1096 ß1 For matrimonial consent to exist, it is necessary that the contracting parties be at least not ignorant of the fact that marriage is a permanent partnership between a man and a woman, ordered to the procreation of children through some form of sexual cooperation.
ß2 This ignorance is not presumed after puberty.
I’d ignore her completely. And, don’t discuss your business with your mother from here on out.