 Regarding natural family planning methods, Pope Pius XII distinguished between engaging in sexual intercourse during infertile days and the specific selection of these days for intercourse.He argued that, if a marital partner entered marriage with the intention to have intercourse only during infertile days in order to avoid having offspring, the marriage contract itself would be invalid.If, on the other hand, the marital partner has intercourse during infertile days only occasionally but not exclusively, then the marriage is legitimate.