Ludwig - //Surely that's what all catholics believe though? - or should believe at any rate. Isn't that their official position? - genuine question. //
Like all religions, and their followers, the rules are, shall we say, 'open to interpretation'.
This means that a Catholic may put money in the Pro-Life collection on Sunday, and then have sexual relations on Sunday night using birth control because he or she does not wish to have a child.
All followers of all religion do this - from the Muslim who shaves his beard, to the fanatic who kills Westerners because they are not Muslims.
Religious doctrine makes no allowance for human nature, but in the real world, almost everyone does, to some degree.