Here, let me try.
We are certainly here, at least in part, to reproduce. It's fun, it's important, it fulfills all sorts of materialistic and theistic agendas.
But fucking isn't part of reproduction. Fucking, in my mind, tends to have more social overtones.
Reproducing involves sex, and ends up with fully grown adults of the species. The male's part varies from species to species - in some, he's essentially there for life, and in others, it's a love-em-and-leave-em affair. Sometimes, like with seahorses, you get weird middle grounds. The female is involved to a greater extent, seeing as how she generally gets the... ah... joy of carrying the babies about.
That's reproduction. In humans, it does (or should) involve a certain level of tenderness and friendship and fucking (people don't suddenly stop being horny after they reproduce). (To all you modernists out there: I think that humans are animals; I also think that they function better with emotional support.)
Fucking is unproductive. Doesn't mean it's bad or anything (see r00k123; it feels quite good), just that it doesn't get you anywhere. The implication that Laurie (described in Rancid_Pickle's wu) is somehow a Good Person because she helps people feel their urges have been fulfilled (or that the job of prostitution in general is somehow laudable), is ridiculous.
Prostitutes and masturbation are kludges. They can be useful but they don't solve the problem. They provide fuckage. The solution is reproduction.
(And fuckage can be problematic, because, at least for me, there's this tendency to treat the "mate" like your mate - probably not a good plan if the "mate", to paraphrase the words of a write-up above, is addicted to hard drugs, has lived an entire life of poverty, has been told that their minds and selves were worth nothing, and has virtually no skills or education. I'm sure there are plenty of very nice prostitutes out there, and a significant number that are mothers, but they aren't the best mother a child could have. They probably aren't going to be good friends, either.)