Knowing/learning about it prior to Marriage is a GREAT idea. There’s no harm in learning the topic before having to employ it.
You have some great answers so far. I’ll add a bit from the Church’s actual language, regarding something you specifically asked:
…yes, I understand that just causes for avoiding should be consistenw tih objective morality. I also don’t really understand exactly what that entails…
…can you guys give me some examples of just causes, preferably not those which are extreme in nature, such as dire poverty or life and death medical conditions, those are fairly straightforward…
"Periodic continence, that is, the method of birth regulation based on self-observation and the use of infertile periods, is in conformity with the objective criteria of morality.” (CCC 2370). It further summarizes: “The regulation of births represents one of the aspects of responsible fatherhood and motherhood.” (CCC 2399).
“The regulation of births, which is an aspect of responsible fatherhood and motherhood, is objectively morally acceptable when it is pursued by the spouses without external pressure; when it is practiced not out of selfishness but for serious reasons; and with methods that conform to the objective criteria of morality, that is, periodic continence and use of the infertile periods.” (Compendium to CCC, Question 497)
“Mastery over drives by one’s reason and free will undoubtedly requires ascesis so that the affective manifestations of conjugal love may be in accord with the right order, in particular with regard to observing periodic continence. Yet this discipline, which is proper to the purity of married couples, far from harming conjugal love, rather confers on it a higher human value. It demands continual effort yet, thanks to its beneficent influence, husband and wife develop their personalities integrally, enriching each other with spiritual values…It favors attention to one’s partner, helps both parties to drive out egoism, the enemy of true love, and deepens their sense of responsibility.” (Paul VI, HV 21 via JP II’s Theology of the Body 59:6)
“ …all pressures brought to bear in limiting "the freedom of couples in deciding about children constitute a grave offence against human dignity and justice" (Pope JPII, Homily In Perth (Australia) Apostolic Pilgrimage to Bangladesh, et. al., November 30, 1996.)
"The judgment concerning the interval of time between births, and that regarding the number of children, belongs to the spouses alone. This is one of their inalienable rights, to be exercised before God with due consideration of their obligations towards themselves, their children already born, the family and society…” (Compendium of the Social Doctrine of the Church, 234).
“Serious motives, such as those which not rarely arise from medical, eugenic, economic and social so-called “indications,” may exempt husband and wife from the obligatory, positive debt for a long period or even for the entire period of matrimonial life.” (Pius XII, Allocution to Midwives)
Hope that helps!