One last thought:
The reason why having children/being open to children is SO CRUCIAL for a sacramental marriage is because children are the natural fruit of intercourse.
The intention to have children within a marriage is important because you can’t separate children from sexual relationships of any kind without introducing toxic mindsets that defy natural law as set down by God. This includes things like sexual exploitation and abortion.
We talk big all the time about how sex leads to pregnancy, about how birth control can’t always prevent pregnancy because of failure rates, about how people should be avoiding sex altogether if they would rather abort than take care of the child that results from sexual activity. We talk so big on the importance of the family unit within society, and the need to accept life as it comes.
You can’t say “pregnancy is a natural result of sex/is a blessing and we should stop anti-child bigotry in our culture” and then turn around and say, “Married couples, who have exclusive access to sexual intercourse, have every right to never have even one child if they wish.” What if they do end up pregnant one day? After all, they entered into a union thinking that they would never have kids and they would never want any, with every right to do so, and with a limiting mindset like that you expect them to actually understand the nature and responsibilities of marital life?
God gave humans the blessing to be fruitful and multiply: whether the fruit for them is one child or many. God condemned a man to death for spilling his seed instead of being open to children. Throughout the Bible, we see men and women being called blessed for having even one child, and St. Paul even goes so far as to say that childbearing is a unique way for women to find sanctification and thus grow closer to Christ. Procreation is revered as a holy, natural, and good part of sexuality all throughout Scripture, and we’re supposed to assume that we can just cut it out if we decide being child-free is better?
God is a God of Unity and Creation. This is reflected within the marital bed in the form of ecstasy between husband and wife, and the possible procreation that can come from the conjugal act between them.
Our God is not a barren god, and those who take on the sacrament of marriage should not act as if that is who we worship.
If you want to have sex but never want any children to result from that, you are not “burning” with sexual desire fit for marriage: you are struggling with the vice of lust. And that lust is not proper for the sacrament of matrimony or for the health of your soul.
Article on Onan’s Sin: https://www.catholic.com/magazine/print-edition/the-sin-of-onan
Humanae Vitae: http://w2.vatican.va/content/paul-vi/en/encyclicals/documents/hf_p-vi_enc_25071968_humanae-vitae.html
The history behind how all Christian churches were against contraception until the 1930s: https://www.crisismagazine.com/2015/reactions-to-the-popes-encyclical-on-contraception
Catechism on marriage, including fruitfulness; feel free to check out the footnotes quoting other writings and Scripture: http://www.vatican.va/archive/ccc_css/archive/catechism/p2s2c3a7.htm