The issue is not to enjoy the act of marriage while intentionally avoiding conception. The method of doing this is irrelevant. The sin is in the will, not in the pill.
If one is abstaining, one is not enjoying the act.
Also, a couple may NOT decide on the number of children before marriage vows as that decision will invalidate their marriage, even if they say, "We will practice virtuous continence after X amount of children." The size of the family must be left up to God at the time of the vow. Most priests do NOT know this. I was not told it during my marriage counseling (over 30 years ago) and no priest I know knows of this. I found it in an old marriage tribunal book. There have been many legitimate annulments granted by past popes on this issue. If, after the vow, there is a problem with the woman's health, or for the case of eugenics or other issues, the couple finds it necessary to NOT have another child, then Castii Conubi allows them to then consider abstinence as a means of avoiding pregnancy. But, it NEVER mentions any sort of indulging in the act while avoiding pregnancy.
The same is for a woman or man who is known to be infertile. If this knowledge is acquired before the marriage vow, then the marriage is invalid. If, after the vows, the couple finds out that one or the other is infertile, this does not invalidate the marriage.