Rhythym/natural planning..whatever
It's a sin in assuming that your own sinful behavior is reflective of the behavior of 500,000,000+ women that you don't personally know.
In the US/developed countries the vast amount of Catholic women are using BC. In 2nd and 3rd world countries probably not because 1.) They cannot afford it and 2.) it is not readily available...when it is they will use it and as they move out of poverty.
Of course in the developed world the Catholic faith is dying except with 2nd and 3rd world immigrants. It is growing in Africa and other such places but those people believe if you rape a baby it will cure aids...so what does that tell you? The Catholic church will die in those countries too once they become more developed.
Totally backward to think condoms/tubal ligations/vasectomies/women priest/altar girls etc are sinful and most people have the same view as I and the church needs to move in the 21st century regarding women taking a more active roll and BC or it will eventually die on the vine.
Actually, the denominations that have "moved in the 21st century" are the ones dying on the vine. (In truth, taking themselves off the Vine is why they're dying.)