STDs and natural selection

Why haven't the laws of natural selection resulted in any STDs that you want to catch? STDs are being transmitted through sex, so you would think that they would be subject to the same laws of natural selection which are evolving the most desirable genes/traits in a species. Wouldn't it be great if there were a BD virus. Every guy in the world would be trying to catch it. Even guys who are innately big would be trying to catch it, because I doubt any guy has ever said "It is big enough". And of course, all of the woman would be trying to catch the Double D variant of the virus.