First off, nobody is obligated to have sex with his/her spouse all the time.
Until about 1600-1700, the constant practice of the West was to designate certain days and seasons of the years as periods when everybody was supposed to be fasting from sex.
For example, Lent and Advent. (And yes, that’s a big reason why the Church refused to hold weddings during Lent or Advent.) Also, pretty much any other day when Catholics were supposed to fast from food or abstain from meat, they were supposed to be abstaining from sex.
Therefore, it was a lot easier for couples to abstain from sex entirely, either for reasons of prayer as St. Paul recommended, or for other serious reasons. And it was reasonably common to have spouses decide mutually to live as brother and sister, often because there was no other way for them to live out their religious vocation, or because they had already had kids and were now called by God to do something else.
On the other hand, the Church is well aware that fasting from sex is difficult on married life, at least for some people. Sexual fasting is no longer imposed in the West. This was done in the hope that married men and women would find it easier to remain faithful and that prostitution would no longer have a reason to exist. (Also, to discourage Catholics from becoming Protestant, and encourage Protestants to become Catholic without too much effort.) But nobody ever said that couples weren’t allowed to sexually fast at all, and they can do it whenever they feel like, and for whatever reasons. (Unless you become a Gnostic couple who hate the human body and the existence of the material universe. Then you two might get forbidden the sexual fasting thing.)
Meanwhile, natural family planning is not something that was envisaged by the early Church Fathers. But abstaining from sex during fertile periods was something understood in theory and it was permissible. (Unlike the various abortifacients and anti-fertility techniques that the Fathers condemned in the strongest terms.) Marriages along the pattern of St. Joseph and the Virgin Mary were considered a bit weird, but were fine as long as you weren’t doing it out of contempt for sex, children, and the material world.
Understanding the Fathers on marriage means understanding the actual words they use. A word for marriage that implies “having sex” is not the same as a word for marriage that implies “partnering up” or “yoked together.” The purpose of a sermon on “Kids are good and the earth was made by God, not some weird Builder or flawed emanation” is different from the purpose of a sermon on “Religious life is awesome! Do it! Here are a bunch of ways that work!” You have to look at things in context.
