I'd say, as long as you know how to avoid sin, you probably don't even need to know a single thing about how procreation works until you are about to marry. Anything else is just a way the devil gets you to dwell on dangerous topics.
If you do this to your sons they are going to be bullied, easily scandalized, and not be prepared to lead their wives in this regard. Five-year-olds who live on farms know about procreation, why on earth would you retard your 18-year-old son and make him weird by sheltering him from such a topic?
If you are easily scandalized or tempted (if you are a teenage boy) you probably should not read the below.Still, no one has answered the question for OP. We have to have an understanding of 1) the ends of marriage, 2) the motives of the act, and 3) the nature of the act.
St. Albert the great lists the ends and motives for us, but for this thread, I will only touch on what he says certainly makes us culpable of grave sin. If the act comes forth with both 1) the end of quieting the concupiscence in mind and 2) at the urge of our vicious nature, then the act, even if completed in the natural way, “the sin is mortal.”
St. Albert then touches on unnatural acts during marriage. "While it remains true that marital intimacies which retain their relation to the primary purposes of conjugal life are never seriously sinful
per se,
yet the manner of their practice may be a sign of concupiscence which is gravely culpable. The attitude adopted in intimacy is a sign of such concupiscence." (Ethics of Conjugal Intimacy According to St. Albert the Great, Clifford).
Here I want to add a small foreword. Even if we decide to disagree with St. Albert here, it at least gives us an idea of what the bias has always been in the medieval Church when it comes to the permissibility of things in the marital act. St. Albert even disagrees with some early scholastics who posited that when the attitude of the marital act is anything outside of a single position, it is always a mortal sin. While St. Albert mentions that this opinion "displeases" him, we can at least see what the bias was. In a similar vein to those early scholastics, some Spanish Catholics in the Middle Ages would cut holes into a sheet and cover their wife with it so as not to incite unnecessary lust. Now ask yourself, what would these early scholastics and Spanish Catholics think if they heard that modern Traditional Catholics were taking the liberty to do whatever they want to do with their wives as long as "it ends up in the right place"? The bias of the Church has always been closer to St. Albert than to Christopher West.
To return to what St. Albert says on this subject: very little. But we can use the above information to inform us that he would obviously be against any sort of action during the martial act that exists outside of the actual marital act. So no, you cannot treat your wife as a prostitute and do whatever you want with her.
All of that being said, moral theology admits of dozens and hundreds and thousands of nuances on a case-by-case basis. If the marital act is directed towards the primary end and the human faculties are not perverted in any way (as they would be in sodomy, making the
substantial nature of the act unnatural), any
accidentals of the act being slightly misdirected do not seem to be enough to make the sin mortal in St. Albert's eyes, but instead may only enlighten us that the motive for the act is teetering on the line between holy and lustful. So, all of this needs to be examined on a case-by-case basis and you will need to let your conscience inform you whether or not you are stepping into the territory of a vicious lust.
What we focus on during the marital act can also inform us of what our motives were for engaging in it. If we are hovering around areas of the marital act that have no direct relation to the primary end of marriage for too long, that can inform us that the ends are being switched in our minds, and that we are giving ourselves up to concupiscence. If there are fleeting gestures that only amount to an
accidental change in the nature of the act, I do not believe you would be gravely culpable (I believe I read something along these lines from St. Alphonsus).
I am not going to go into gross specifics, but I do not think men should get scrupulous about small touches, kisses, and other things done during the marital act which are not
substantially unnatural. If you read through my post, I think you would get the gist enough of what specific things may be permissible and what are not without me having to spell everything out.
I think we all know what is an is not permissible in the bedroom. We just want to convince ourselves that we can take more liberty than we should by digging up what this or that "moral theologian" said on the subject. Stick to the great Saints. Stick to St. Thomas, St. Albert, and St. Alphonsus on this and you can't go wrong.