Some things are so obvious that we take them for granted. We don't feel that we need to “argue” them because we casually assume that everyone “knows they are true.” The old Frank Sinatra song comes to my mind: “Love and marriage: go together like a horse and carriage . . .”
But wait! Today they don't always go together, and we see multiple thousands of couples living together without being married. This seeming common sense “truism” has been largely lost, in terms of being the norm in our society. And of course, now even marriage as solely between a man and a woman is up for question.
Marriage and procreation is another such “obvious” pairing that is increasingly denied today. What once was understood by virtually everyone (certainly by all Christians), now no longer is, and is rejected outright by more and more couples.
So we Catholics and other conscientious parents concerned with educating children in traditional morality need to go back to the basics and cite inspired Holy Scripture in order to reinforce basic moral truths: sort of like learning our “ABC's” all over again.
The Bible is quite clear on this point, and – as in many cases, including even the very existence of God – assumes its truth: so much so, that it doesn't bother to explain why it's true. In fact, the very first command to human beings recorded in the Bible is “Be fruitful and multiply” (Gen 1:28; RSV).
We might also note (taking into account common complaints and misunderstandings about Catholic teaching), that what the Church calls the “unitive” function of marriage is also alluded to early on in Genesis, where God says: “It is not good that the man should be alone; I will make him a helper fit for him” (2:18). Marriage is not only for procreation; nor does it consist only of pleasure and companionship, but both. Yet procreation is the fundamental purpose and essence of marriage (and the “relations” within it): why it exists in the first place.
The notion of fruitfulness and multiplying is repeatedly presented as a blessing in the Bible (Gen 9:1, 7; 35:11; Ps 107:38; Is 48:18-19; Jer 29:6; 30:19-20; 33:22; Ezek 36:10-12; Dan 3:35-36; Baruch 2:34).
The ultimate cause of this increasing is God, not man and his choices, though human beings indeed “work together” with God, always enabled by His grace (cf. 1 Cor 3:9-10; 15:10; 2 Cor 6:1; Phil 2:13):
Genesis 28:3 God Almighty bless you and make you fruitful and multiply you . . .
Deuteronomy 7:13-14 He will love you, bless you, and multiply you; he will also bless the fruit of your body and the fruit of your ground, your grain and your wine and your oil, the increase of your cattle and the young of your flock, in the land which he swore to your fathers to give you.  You shall be blessed above all peoples; there shall not be male or female barren among you, or among your cattle.
Psalm 105:24 And the LORD made his people very fruitful, and made them stronger than their foes. (cf. 115:14)
This is the natural order of things. We've come from that to a strange place where three or more children is considered “too many.” My wife and I ran into two nice ladies once at our local Highland Games, and we mentioned that we had four children. One of them immediately shot back with, “oh, you must be Catholic.” I didn't think fast enough at the time. Instead of smiling sheepishly and saying, “yeah,” I should have replied, “we have four children because we're married.” That would have been much more interesting and could quite possibly have led to a fruitful discussion (no pun intended).
The Bible also uses the phrase “opened her womb”, in reference to God with regard to both Leah (Gen 29:31) and Rachel (Gen 30:22). It's not just a trite observation on the level, say, of a popular greeting card. It's a profound truth. Real cause and effect is in play. Other passages teach the same notion:
Joshua 24:3-4 Then I took your father Abraham from beyond the River and led him through all the land of Canaan, and made his offspring many. I gave him Isaac;  and to Isaac I gave Jacob and Esau. . . .
Ruth 4:13 So Bo'az took Ruth and she became his wife; . . . and the LORD gave her conception, and she bore a son.
Moreover, Scripture describes God as the primary creator throughout the process of conception and childbirth: “Thy hands fashioned and made me” (Job 10:8); “Thou didst clothe me with skin and flesh, and knit me together with bones and sinews” (Job 10:11); “In his hand is the life of every living thing” (Job 12:10); “did not one fashion us in the womb?” (Job 31:15); “. . . the LORD who made you, who formed you from the womb . . .” (Is 44:2; cf. 44:24).
Granted, the parents and natural processes are also causes. But we mustn't remove God from the equation, or in effect, “tie His hands” when He would will that a child be born, and we “overrule” Him, so to speak. This is the evil in contraception, or what is known as the “contralife will.” But when we are open to life, we freely work together with God.
The many biblical passages about children (especially, many of them) being a great blessing suggest procreation as the central purpose of marriage – certainly the very opposite of a “contraceptive outlook” (see: Gen 17:16; 33:5; 1 Chr 25:5; Ps 127:3-5.
The world is turned upside down: in the wrong direction. The ugly fruit of secularism is apparent. Thus, our task is to present traditional moral teaching to our children and anyone else, when the topic comes up, and to explain why we believe as we do: to defend what used to be patently obvious to one and all.