Do condoms cause abortions? No, they are a "barrier" to the fertilization of the egg. If a married couple opts for condoms to regulate - though in many cases, it doesn't - the spacing of children into their family, is their use of condoms wrong? If condoms in this case are wrong, then how is NFP (approved by the RCC) any less illicit? In both cases the intent is to prevent fertilization and the start of a pregnancy.
There are certain things that God has seen fit to leave up to a person's own liberty, the ability to make personal choices about things that specifically affect him or her. Some things are between us and God and aren't anyone else's business. But where God's word IS specific, we have no excuse but to follow the principles laid out.
Condoms and NFP are both immoral. Condoms and NFP are both OK.