And then, come together again, lest Satan tempt you ... for we are not ignorant of his devices. Like fasting, this kind of abstinence is viewed as a temporary condition. To make it permanent is to sin against the spouse, and the God Who ordained and blessed the marital union as normative.
It is *accidentally* temporary, not *essentially* temporary. It is *accidentally* temporary because typically one or the other spouse is subject to temptation. But for those not subject to such temptation, abstinence need not be temporary, otherwise a married couple would be obligated to engage in sexual intercourse long after both spouses had, on account of advanced age or some other condition, ceased to have a desire for sexual intercourse. And therefore, since Mary and Joseph were not in such a state as to be subject to this sort of temptation, they could, without any sinfulness, or injustice toward one another, abstain from sexual intercourse permanently.
-A8