Very well summarised, jmjZelie! Aquinas was the great natural law philosopher with regards to our approach to morality and it’s that aspect that inspires my faith in his contribution to Church teaching.
It’s too unreasonable to believe that reproduction should only depend on the fertile status of a creature and the sexual capacity to create a new life. That’s what seems to be behind the belief that the default of a married couple is to keep on having children one after the other based on the sexual desire of the couple (generally speaking, it is the sexual drive of the male that dominates and that’s the flaw in the patriarchal cults that so often leads to abuse of females and children in my opinion.)
The mores of reproduction can be seen to vary in nature such as when female creatures as a group, hide themselves or naturally shield themselves from sexual encounter with males. These responses are regarded as natural phenomenon for the benefit of the species.
In human beings our capacity for morality has to be based on our deeper understanding and appreciation of ‘relationship’. Not just empathy in the couple relationship in planning family but appreciation for our role in wider society and in the whole environment of the planet and how that is understood by us through our human nature!
Yes, we are not merely animals, whose reproductive instincts are governed solely by fertility and the urge to mate. Marital relations should be human in nature, which means that we must choose whether or not to engage in marital lovemaking. It can still be called instinctual, but it is human instinct, not animalistic. Human instinct includes things such as social cues, quality of relationships with others, quality of interactions with others, seasons of life, things irrelevant to animals such as privacy and emotional intimacy and time and comfort, physical wellbeing mental wellbeing…well, I could probably list a whole page of differences between animal instinct and truly human instincts.
I have read some wonderful treatises recently of how to handle any mismatch of libido of married couples (so common as to almost be the rule). It suggests that the best way to work out the mismatch and to keep the marital act a truly human and loving one, is for both spouses to meet in the middle. The higher libido spouse restrains him/herself somewhat so as not to fall into using the lower libido spouse. And the lower libido spouse works to be receptive more often than their own libido might prompt them. And of course, when marital relations are not a good idea for reasons of health or privacy or whatever, practicing restraint without resentment or bitterness is praiseworthy.
I know libido is not the point of this thread, but I just wanted to respond to a couple of the points in your reply.
With respect to the overall point of this thread, I am probably a traditionalist at heart. I tend toward that side of things naturally. But, and this is a big BUT (

), I have never associated traditionalism with legalism. I think some people today, who perhaps are not traditionalists, might make the mistake to think that people who gravitate toward the traditional side of the faith are also legalists. In my experience, that is not so. There are legalists in every pocket of the Church. Some of them manifest as minimalists (advocating doing the absolute minimal to attain salvation) and some of them manifest as those who tie up burdens upon the backs of others without lifting a finger to help them. There are probably others, too, that I am not aware of. It is the middle path that we must strive for. We should know our Faith and live out our Faith with as much joy as we can muster (and I say this as I am personally going back through a particularly joyless period), and then we practice the corporal and spiritual works of mercy with love of God and love of neighbor for the love of God as the foremost thought in our mind. If we have no intention of understanding the Gospel as Jesus Christ MEANT it, and if we refuse to even attempt to see our neighbor as God sees them, then we had better zip our lips until we change our own bad attitude.
On a forum like this, it is possible to debate ideas in the abstract and it is possible to give encouragement and general advice, but it is CATHOLIC Answers, NOT Catholics ANSWER. Ergo, our opinions must not be presented as if they have magisterial backing, unless we are actually presenting magisterial teachings, ie the Catechism of the Catholic Church or other magisterial documents.
Sorry for the long rambling post. I think that just writing all this out has been helpful to me to crystallize my thinking about these ideas. I hope it has been helpful to others who read it. God bless!