Err, if use of contraceptives isn't a person trying to prevent procreation, what is? Now, if you're using a drug with contraceptive side effects to treat an illness, that's different. I do feel the person is morally obligated to abstain from relations while on said drug, however, because of the possible abortive effects contraceptives possess.
In other words, if a man or woman is recieving a treatment for, say, cancer that renders them sterile, there is no moral problem with them carrying on relations within marriage as there is no possiblity of conception or abortion, and the effect is the unintended consequence of a life or death decision. On the other hand, if a woman is taking the Pill as a remedy for unusually heavy menstruation (hopefully after being very sure it won't mask symptoms of a greater problem), she should abstain from relations as there is still the possibility of conceiving while on the drug, and the resultant fetus being aborted due to the unhospitable environment birth control creates in the womb.
Err, if it's not a specific response to a specific command of God.