That paragraph of HV (par. 15) indicates that therapeutic means (medication, for instance) may be used, even if impedes procreation. However, the intent cannot be to impede procreation. The question is whether one can have conjugal relations when the procreative means is impaired. Since the act would be rendered non-procreative, it would not be permitted.
Look at the prior paragraph of HV regarding intent:
Neither is it valid to argue, as a justification for sexual intercourse which is deliberately contraceptive, that a lesser evil is to be preferred to a greater one, or that such intercourse would merge with procreative acts of past and future to form a single entity, and so be qualified by exactly the same moral goodness as these. . . . it is never lawful, even for the gravest reasons, to do evil that good may come of it (18)—in other words, to intend directly something which of its very nature contradicts the moral order, and which must therefore be judged unworthy of man, even though the intention is to protect or promote the welfare of an individual, of a family or of society in general. Consequently, it is a serious error to think that a whole married life of otherwise normal relations can justify sexual intercourse which is deliberately contraceptive and so intrinsically wrong.
This demonstrates that for whatever reason, you cannot engage in an act which contradicts the moral order - contraceptive sex, rape, adultery, homosexual acts, etc. The fact that you benefitting your marriage, or you have a marriage that is procreative, does not excuse any contracepted sex.
The pill is not sinful if used therapeutically. However, contracepted sex is always sinful, because it renders procreation impossible.