"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."
Even Christ was tempted by Satan. Even if we agree that Mary and Joseph abstained, why should we believe that they were not subject to sexual temptation?
Simplifying a bit, there are two ways to be tempted: externaly or internally. To be tempted internally means that one's own disordered desires play a role in the temptation. Jesus had no disordered desires. Therefore, the devil could truly tempt Him externally [the way Adam and Eve were originally tempted], but not internally, i.e. not by making use of His [Jesus's] disordered desires, since Jesus had no disordered desires.
So the fact that Jesus was tempted [externally] does not nullify my previous point about permanent abstinence during marriage being permissible when neither spouse is subject to temptation, because there I was talking about *internal* temptation. And Mary did not have any disordered desires, and Joseph likewise, presumably at least did not have disordered sexual desires.
-A8