IT WAS over twenty years ago that a major sex study disclosed what was then a rather startling finding: lesbian couples engaged in sexual activity significantly less frequently than did both heterosexual and gay male couples …

… Conceptions of sex, after all, vary from one cultural epoch to the next, but also during the lifespan of an individual. The urge to reproduce is an arc, you might say, that rises into the sky, like a rainbow, and then returns to earth. Once it occurred to me to ask a friend who’d sired six children if this had lessened his sex drive, and it made no sense when he said No. …