Nobody seems to be able to give a consistent unflawed reason why "contraception is a perverted use of a human gift" and timing one's sex with the aim of achieving the same is not beyond "because we say so".
Perhaps because UB's two threads have both gone into the direction of contraception, I should just link to my explanations
over there. But I'll also post some of the briefer comments I think may summate Catholic teaching on the matter. I think accusing the Church of "because we say so" does zero respect to the fact that the Catholic Church has rationale for teaching as She does. That's really an impoverished "James White" type of an argument that he would never use against, say, St. Paul for teaching something authoritatively.
But in short, contraception is wrong, not just because God said so (ahem, cough, through the Catholic Church as we understand) but because of its very nature against the gift. Also, I think I gave you a very reasoned answer as to why it is wrong vs. abstinence in post 51, too.
In the other thread I wrote this: "It is sinful because it is a violation of the natural law and an affront to the gift of sexuality, frustrating a part of the giving of the self which sex is."
I think if you read through the links I gave in post 47 in this thread of various Magisterial documents, especially Humanae Vitae or JP2's Theology of the Body, you will find much more scholarly and in depth discussion on the very meaning of being human and the gift of sexuality we have been given, over and above even the angels.
Be assured, no one is attempting a "because we say so" attitude with no merit for the teaching. The Apostles, even with their authority to do so, did not have that attitude. There really is a theology behind it not pulled out of thin air despite the flippant comments by some in this thread and elsewhere in the contra-Catholic world.