In the brave new world of post-Soviet sexuality, Sergei Agarkov is a pioneer. And Muscovites kept up at night by nagging questions and concerns of a decidedly delicate nature are no longer alone. Agarkov, a ruddy, heavy-set man, is the director of Intim, a Moscow salon that has been selling contraceptives and sexual aids for more than two years. A branch was opened a year ago in St. Petersburg, one block away from the Moscow train station. Business has been brisk, but it has not been risqu?. ""The majority of Western stores that disseminate sexual products are oriented toward pornography or sado-masochism,"" Agarkov said. ""Our store is a bit different from Western sex shops in that it is oriented more toward medical help."" Located in the Medical Diagnostic Center for Medicine and Reproduction, a northeast Moscow clinic that offers treatment for sexual problems and infertility, the unobtrusive store is easy to miss. Once inside Intim, however, visitors are not likely to forget what they have seen.