Really? Really? The purpose of sex within a marriage is NOT deliberate stimulation of the genital organs in order to derive sexual pleasure. Not even close, not even a little bit. That’s a little like saying we eat food because it tastes good. But your characterization is even worse than that… Its not just, as secular people might say, that sex is an unifying act of love. It is, sure. But I have sex with my wife as an expression of our unity which reflects the unity of Christ and His Church. I give myself to my wife, and she to me, as Christ gives Himself to the Church. Sex within my marriage is a most Holy Office to which I have been called. And even though the pleasure is not even remotely the point, I will also say that the pleasure in Holy sex is of a completely different character than masturbation. The carnal aspect of the pleasure pales in comparison with the spiritual aspect.
In this way, I almost don’t think its quite right to say Catholics oppose masturbation per se. Its that we know the real alternative to be so unimaginably better that we cannot bear to be distracted by a cheap counterfeit.