There are many sources out there…here is a brief one:
Compendium issued by Pope Benedict XVI (with emph)
**
497. When is it moral to regulate births?**
2368-2369
2399
The regulation of births, which is an aspect of responsible fatherhood and motherhood,
is objectively morally acceptable when it is pursued by the spouses without external pressure; when it is practiced not out of selfishness but for serious reasons; and with methods that conform to the objective criteria of morality, that is, periodic continence and use of the infertile periods.
498. What are immoral means of birth control?
2370-2372
Every action - for example, direct sterilization or contraception - is intrinsically immoral which (either in anticipation of the conjugal act, in its accomplishment or in the development of its natural consequences) proposes, as an end or as a means, to hinder procreation.
vatican.va/archive/compendium_ccc/documents/archive_2005_compendium-ccc_en.html
NB: NFP is a form of “periodic continence”.
Contraception is an
immoral means of regulating births.