I could be wrong, but I don't think the Church teaches that sex is for only one purpose, i.e., reproduction. The modern Church has gotten away from that.  So I think you've based most of your argument on a strawman.  

And given the trajedy that non-marital sex can visit on the human race---look what it's doing to large segments of the black population and increasingly large segments of the hispanic and white population---who would reasonably argue that non-married sex is, in balance, a good thing? The Jews had it right thousands of years ago. Only modernity could get it as wrong as we have it now---children raising children, raised in the belief that we are not free, but instead slaves to our biological urge. Gays don;t even have that excuse, as there's nothing biologically urgent in their case. Sexuality is too dangerous NOT to be governed by Natural Law.  The poor pay the price of your moral code.