“The Church, nevertheless, in urging men to the observance of the precepts of the natural law, which it interprets by its constant doctrine, teaches that each and every marital act must of necessity retain its intrinsic relationship to the procreation of human life.” (Humanae Vitae, 10)
The reason artificial birth control is immoral is because it is an act, that is by its instrinsic nature, opposed to natural law, whereas abstinence is not.
Abstinence from sex is not opposed, by it’s instrinsic nature, to natural law. Responsible married couples understand that abstinence is required (e.g. “Honey I have a headache”). The act of NOT having sex is not instrinsically evil (otherwise, Jesus and Mary have some splainin’ to do).
Actions which intentionally prevent the release of ovum to conceive are by their intrinsic nature, opposed to the purpose of the marital act. Likewise, actions which intentionally prevent the release of sperm to conceive are by their intrinsic nature, opposed to the purpose of the natural purpose of the marital act.
Abstinence from the marital act is not the same as willing the purpose of a marital act and not willing it at the same time.
Marital acts during infertile periods is not, by the intrinsic nature of the act itself, opposed to conception. In other words, the marital act itself retains its intrinsic relationship to procreation. The lack of contraception is not by acts opposed to natural law, but due to the natural lack of fertility. The key word in Catholic moral theology regarding this teaching is “intrinsic.”