A recent study regarding mature women's turn-ons is, somewhat ironically, published in a journal whose title is an instant turnoff: The Journal of the American Geriatrics Society. In any event, a U.C. San Francisco survey published this week plumbed the desires and activities of more than 2,000 women aged 45 to 80 (you try telling a 45-year-old woman she's "geriatric" -- be our guest) and found that 60 percent had been sexually active within three months and nearly half -- 43 percent -- reported