Answer by Ed G.
There was a thread on this very topic (actually as on offshoot of a thread on contraception), discussing why artificial contraception is bad while natural family planning is acceptable. Mama Toad and Katievs, among others, were adamant that sex within marriage is a-ok and that NFP was allowed; I think the main distinction they rested on was that marriage itself is ordered toward procreation even if individual instances aren't. I, and to a lesser degree Tomm DeSeno, were less certain of all that; it seems to me that intentionally avoiding procreation with strategic abstinence (NFP) puts the resulting sex acts firmly into the non-procreative camp and therefore presumably not to be condoned.
It seems that we have a contradiction in the books: on the one hand the Church teaches that marital sex acts must be both unitive and procreative (see Tom Lindholtz references above), and on the other hand we have Pope Paul VI endorsing the strategic abstinence which results in sex that is intentionally not procreative (again see Tom Lindholtz references above).
I truly don't know how to reconcile these two views, so I don't try. The card says Moops, so I'll take it that my wife and I are safe in our expressions of marital love even when we're intentionally avoiding the procreative power of our union.