I think you’re conflating two issues–the desire to avoid children and the means of doing so. It is not necessarily a sin to want to avoid children (it can be). The sin with regard to contraception has nothing to do with your motives, but with deliberately impeding the natural end of the act itself.
While it is the general mission and duty of married couples to bring forth children, there can be good reasons to regulate or limit births. If a couple chose to avoid having children for selfish reasons, say, so they could instead focus on cultivating their Lamborghini collection, it would be a sin even if they practiced complete abstinence.
But when there is a justifiable reason to avoid having a child, then we have to evaluate the means of doing so. Total or periodic abstinence (NFP) is a morally licit means of doing so.
What is a sin, no matter the motive, is “any use whatsoever of matrimony exercised in such a way that the act is deliberately frustrated in its natural power to generate life.” Abstinence–total or periodic–does not involve this.
From the CCC:
2367 Called to give life, spouses share in the creative power and fatherhood of God.154 "Married couples should regard it as their proper mission to transmit human life and to educate their children; they should realize that they are thereby cooperating with the love of God the Creator and are, in a certain sense, its interpreters. They will fulfill this duty with a sense of human and Christian responsibility."155
2368 A particular aspect of this responsibility concerns the regulation of procreation. For just reasons, spouses may wish to space the births of their children. It is their duty to make certain that their desire is not motivated by selfishness but is in conformity with the generosity appropriate to responsible parenthood. Moreover, they should conform their behavior to the objective criteria of morality:
When it is a question of harmonizing married love with the responsible transmission of life, the morality of the behavior does not depend on sincere intention and evaluation of motives alone; but it must be determined by objective criteria, criteria drawn from the nature of the person and his acts criteria that respect the total meaning of mutual self-giving and human procreation in the context of true love; this is possible only if the virtue of married chastity is practiced with sincerity of heart.156