It is not hidden knowledge.
There is huge amount of confusion.
Just examples:
1. How we should take care of our and our spouses needs?
2. What are good limits / borders in marriage: when husband or wife are allowed to expect anything from other side?
3. Stereotypical: when she says no she means yes. I know it is just foolish saying but for many young men and women alike from broken families (my filter) there is a lot of confusing trivia about men/women relationship which does not bring any help to anyone.
4. Growing expectation from women for men to be of help in the house with cleaning, washing, babysitting, maternity leave etc - well it is not bad per se but - first of all men need to become a men. If not there would be a lot of confusion and mess.
5. Porn culture: growing illusion among men that there is some kind of "sexual heaven" that one may fulfill all desires with woman here and now without any knowledge of reality of real woman, of real man, real me. How to converse, how to see what to look for, and also very important to stop exciting instinct and waste energy on illusions and chasing somebody that does not exist. Sex is confused with love - obvious. As in general society at least in Poland in recent years is more detached from Church then concept of other love, eternal love, love with sacrifice is going to archeological concept. We have now TV, Netflix, Youtube, music that my tell us stories about love which is becoming more and more materialistic then ever (my feeling).
6. How we may support each other in becoming best versions of ourselves: how may I support my woman, and how my woman may support me - that is everyday life knowledge that would be so cool to understand, agree on before getting into relation.
It is not hidden knowledge but we are not taught that. We learn how to get the job (not even that properly) but things like marriage, upbringing, self education, self discipline, aims, morals, values are very often part of religious upbringing then standard education. One is really happy to have wise surrounding, wise parents, family who may talk at the table about marriage, relations, solving problems etc.
Women are expecting good man - but in my feeling many of good women do not realize how tremendous influence they have on what future generation of men they create. Similar can be said about men - what kind of power we have to create women that we so desire. But along our ways there is a lot of temptations of using this power for self benefit, self pleasure. By men and women alike.
So, maybe it is not so much hidden knowledge but it is a lot of skills and understanding required to be succesful in creating healthy, supporting ground for the couple and kids and by the same time understand what are consequences of not doing right things. I see many many trials and errors and in general I may say that it is not looking good.