Did Sister Mary Anthony have in mind ladies enjoying sex in marriage on all occasions. The corollary of the position that she required her bumps feeling by a caring counsellor is that ladies should not have sex unless they are enjoying it.
Sex in marriage has a range of possibilities. And it depends how the word "sex" is defined. Sex in marriage occurs, it is said with decreasing frequency over time, on a fairly large number of occasions during the course of a Catholic marriage, which is what Sister Mary was presumably referring to as opposed to the number of variations available to non-Catholics. It would be unreasonable to expect her to be referring to anything else.
What the dear, devout nun said on this matter seems to have been one of the things you remember after all this time. But she is faced with the alternative--not being devout. Now osso--not being devout is a rather open ended situation. Do you draw any lines in the sand on not being devout?
Would the parents who sent you to the school expect the nuns to advise their innocent and fragile little charges on aspects of not being devout?
How you got an A+ in logic is inexplicable if you cannot consider Sister Mary's alternative.
I have been told by a number of ladies that it being a sin adds significantly to the enjoyment.