And let's be frank. Evolution is all about sex, genes, and breeding. The latter two topics might not sound sexy, but sex has everything to do with why you and I are here, and why you and I behave the way we do. Sex is ultimately why I write this blog (narcissism, fame, my picture on computer screens far and wide across the world - all should increase my local fitness and enhance the fitness of my children if all goes well, or at least fools my brain into thinking so). I've been reading The Red Queen: Sex and the Evolution of Human Nature, so genetics is a bit on the mind. I'm a psychiatrist, in America a discipline mostly inherited from the theoretical perspective of one dirty old man. Nothing in human evolution makes sense without considering sex. In my mind one of the positives of a paleolithic style diet is the anecdotal reports of (mostly) increased sexual drive. Vegetarians (in anecdote also) will often report a decreased sex drive.
In the comments for Jamie's post I quipped that decreased libido was probably a bad sign if it seemed to be caused by diet. If your body doesn't think you are fit for reproducing, it won't send you signals leading that direction, after all. And if your body doesn't think your diet is good enough to support little ones, it makes sense that your diet is not likely optimal for human health.
So that's the sex part. What about the drugs? Well, as a psychiatrist, I happen to have a lot of experience with drugs that decrease libido. And I hate being a hypocrite. Some of the data can be clouded by the fact that depression itself causes decreased libido. But the fact of the matter is that SSRIs and their cousins seem to cut down sexual desire and other phases of sexuality. With SSRI's it seems that the drugs cause a direct pharmacologic "anti-viagra" in men, and more commonly in women a decreased sexual interest caused by mucking around with certain varieties of serotonin receptors. These are different problems than the low-protein diet - which seems to affect sex hormone production (here's an explanatory bit from a book about pigs, anyway).
Ideally, we could figure out a way to get depressed and anxious people feeling better without messy pharmacologic interventions. I would start with therapy, exercise, and a primal-style diet (Mark Sisson's 100-150g carbs a day with a pass on the starchy tubers as part of the non-toxic carbs, to be crystal clear).
And why sex (and males) in the first place, when parthenogenesis is just so much neater and cleaner and doesn't require all this vast effort to blog, not to mention black eyeliner, platform black suede boots, and haircuts? The Red Queen: Sex and the Evolution of Human Nature (pg 86) has the best argument I've heard - disease.
Sex is about disease. It is used to combat the threat from parasites. Organisms need sex to keep their genes one step ahead of their parasites. Men are not redundant after all; they are a woman's insurance policy against her children being wiped out by influenza and smallpox (if that is a consolation). Women add sperm to their eggs because if they did not, the resulting babies would be identically vulnerable to the first parasite that picked their genetic locks.
It is not an accident that the most polymorphic genes are the immunologic histocompatibility genes. Meaning that each generational variability seeks above all to outwit the pathogens that feed on us. Sex is the best generational bet to supply different MHC genes to confuse the viruses, bacteria, parasites, and fungi. There are no long-lived species without sex. Which, using somewhat un-rigorous thinking, means I will never be a vegan.