It represents the separation of the pleasurable aspect of a natural act and its natural end, i.e., nutrition, or in the case of Onanism, masturbation, and birth control, reproduction. Therefore; ANY sexual intercourse, WHERE ONE KNOWS THAT CONCEPTION IS NOT POSSIBLE would be considered sin.
That 'seed' is SURELY wasted and spilled on the ground!
Contraception is "any action which, either in anticipation of the conjugal act [sexual intercourse], or in its accomplishment,
or in the development of its natural consequences, proposes, whether as an end or as a means, to render procreation impossible"
(Humanae Vitae 14). This includes sterilization, condoms and other barrier methods, spermicides, coitus interruptus (withdrawal method), the Pill, and all other such methods.