The words are translations from another language. (Usually Latin, but I think the Catechism was originally written in French.) In trying to find the word in other documents just now I found the English translation in Humanae Vitae point 16 paragraph 2 commonly uses the word “serious” but I also found it translated as “well-grounded”. I know I’ve seen the word “grave” used also, but don’t remember which document or which translation.
"…If, then, there are serious motives to space out births, which derive from the physical or psychological conditions of husband and wife, or from external conditions, the Church teaches that it is then licit to take into account the natural rhythms immanent in the generative functions, for the use of marriage in the infecund periods only, and in this way to regulate birth without offending the moral principles which have been recalled earlier."
Here’s the Latin
Si igitur iustae adsint causae generationes subsequentes intervallandi, quae a coniugum corporis vel animi condicionibus, aut ab externis rerum adiunctis proficiscantur, Ecclesia docet, tunc licere coniugibus sequi vices naturales, generandi facultatibus immanentes, in maritali commercio habendo iis dumtaxat temporibus, quae conceptione vacent, atque adeo nasciturae proli ita consulere, ut morum doctrina, quam modo exposuimus, haudquaquam laedatur.
I don’t think it’s entirely just semantics to ponder what exactly the Church means. It’s clear that married couples should have some reason(significant/just/serious/grave/well grounded/whatever) to space births or avoid pregnancy. The Church doesn’t go into great detail defining the ____ reasons people can regulate birth, so we are left to reflect on what that means for us and our spouses in prayer.
Many seem to forget the little detail that the Church teaches we need a just/serious/grave/well grounded/whatever reason, and may think “because we don’t want another one” is a good enough reason to avoid another child. Many priests seem happy if couples use NFP instead of artificial contraception so they don’t question why a couple is avoiding. Some people mistakenly believe the Church “promotes” avoiding children through NFP, and that also adds to misunderstanding by people who use contraception.
I don’t think this is merely a question of semantics; I think this part of the Church teachings is not well defined or understood in modern English, so it is often overlooked or ignored.