Called so because it is the only form of contraception endorsed by the Catholic Church.
Current methods of Natural Family Planning (non-chemical and non-barrier contraception), when used correctly and consistently, have been shown in several peer-reviewed studies to be over 99% effective. This is similar to the efficacy of 'the pill.'
It should be noted that the use of such a term often indicates it's user has a bigoted and intolerant attitude against people who practice a religion they do not agree with.
They have six kids? They must be playing Vatican Roulette.