The principle is similar to, if not identical with “double effect.”
Contraception is having sex with the deliberate end of not having children by using artifical means to break or disconnect the means from the end.
NFP means working within the natural parameters to legitimately use an act to the extent that specific ends (procreation) are not necessitated.
The intended end (physical union or bonding) is legitimate within the natural and ethical parameters of how the means (sexual act) can be used. There is no intent to thwart procreation since procreation would not occur, in any case, given due diligence.
With contraception the intended outcome is sex by deliberately targeting or making the end (procreation) artificially impossible.
It could be argued that if God simply intended procreation to be the purpose of sex, he would have made conception the result of every act of sex. And if he intended mere pleasure/bonding to be the purpose, then sex would never end in pregnancy. By making procreation somewhat improbable – but under the auspices understanding, reverence and love – then a case could be made that responsible sex was his intention for BOTH the ends of procreation and for fostering the loving bond between the complementary pair. Not one nor the other, but BOTH together.
Responsible “love making” in the sense that both its ends are to be held in consistent balance with each other: loving unity to support procreation and procreation to add or create tangible, subsistent meaning (new life) to loving unity.
I recognize I am just rambling here – but there is a point in there somewhere, just inelegantly made.