If one of the purposes of the sexual powers is procreation, doesn’t it follow that lifelong abstinence is contrary to natural law?
Good question, but the answer is no, not by a mile. Consider an analogy. Eating a meal is good, but one doesn’t have to eat all the time. For example, you might fast to lose weight or to spend more time in contemplation of God. What natural law requires is not that you be constantly having dinner, but that when you do enjoy it, you do so in a way that doesn’t drag its nutritional purpose through the mud. For example, it would be wrong (and by the way disgusting) to eat until the bursting point, then purge in order to eat some more.
In the same way, the sexual union of the husband and wife is good, but one doesn’t have to make love all the time – or even marry at all, since sexual intercourse isn’t necessary for life, as eating is, and as good as marriage is, it isn’t the highest thing. For example, you might vow a lifelong fast from sexual intimacy for the sake of the consecrated religious life. What natural law does require is that if you do practice sexual intercourse, you do so in a way that respects its inbuilt procreative and unitive meanings. For example, it would be wrong to use artificial contraceptives (which is like purging during dinner), or to make the act of intercourse an exercise of cruelty.