I think this is the perception of most in our culture. If they are Christian, they feel that they have fulfilled God's command by having children. And the second, in regards to material possessions is the predominant view.
There are two main theological issues at hand here that I would like to address here:
1. Fulfillment of God's Law
First off, no, God does tell us how much we must multiply by, but neither does he give to us to say when we have "done enough." So you say, you've been fruitful, you have multiplied. Good for you, you Pharisee who looks to the law as yours to fulfill and earn God's grace. The reality is, God gives to us to be fruitful and multiply. And this is not only a "pre-fall command" (as I have heard others say) but one that is given again after the flood (Gen. 9). Being fruitful and multiplying, i.e. having children, is not about fulfilling God's law. We will never fully fulfill God's law, for we are sinners. For this reason, God sent his Son, to redeem us who are under the law (Gal. 4). We must stop looking to ourselves as completing God's work in us, for that is the way of the Pharisees and the Romanists and many American "Evangelicals". But God's law is fulfilled in Christ, we are recepients of his good and gracious gifts... which brings us to:
2. 1st Article Gifts
To be fruitful and to multiply is a part of God's good and gracious gifts to his people. As repeatedly discussed on this blog and elsewhere, children are gifts of God. Rarely will a Christian argue this point. But what about not having more children so that I may enjoy the finer things in life? Regardless of one's words, looking to earthly and material goods for oneself is nothing short of idolatry. You either idolize yourself in your own fulfilling of God's command or you idolize money and possessions for yourself. God has not promised that you will be provided with the finer things of life, you may never own a new car, you may never be able to eat steak and seafood on a regular basis, you may never even have an Armani suit, and you may even never get to go on that fabulous vacation you dreamed about. Not that there is anything inherently wrong with such things, but these are not yours by birth right, they are additional gifts. God "richly and daily provides me with all that I need to support this body and life." (1st Article of the Creed, Small Catechism)
In our society, the definition of needs is so highly scewed. The arguement of prohibiting children from the standpoint of gaining earthly possessions is idolatry and shows a lack of faith in the 1st Article of the Apostle's Creed, that we will provided with all that we need to support this body and life. Oh and by the way, did you notice that one's wife and children are part of these same 1st Article gifts
We should not be as fundamentalists, who hinge salvation itself on the fulfilling of God's laws, but neither are we given to reject God's gifts. No other gifts mentioned in the 1st Article of the Creed as listed in the Small Catechism, would anyone reject, except that of children. No, salvation does not depend on it entirely, but hearing God's Word on the issue, receiving God's gifts, earthly gifts as well as the gifts given to us in Christ, "for all this it is our duty that thank and praise, serve and obey Him. This is most certainly true."