I thin it's because often the religion is difficult to follow 100% in this day and age.
For instance, if there was a Catholic couple or a couple who practised another religion that doesn't believe in birth control, they should really not indulge in sexual activities. Although they may be married, they may not be able to afford to bring up a child at that time.
The responsible thing to do therefore, would be to not get pregnant.
The fact is, Catholicism dictates that all forms of birth control are wrong.
The couple cannot therefore have sexual intercourse for fear of pregnancy - according to their religion that is. So they may choose to disregard that and use contraception.
They still believe they're a good God-fearing couple - as well they may be, but they are disregarding at least one of the rules of their religion.
Religion rarely changes with the times and in the above instance, I feel that particular rule is not workable for all practising Catholics and may answer - at least in part - why some may choose to disregard certain religious laws.