IN Grandmother's day, underwear was not only something never to be seen, but something never to be talked about, either. "Unmentionables" was the euphemism for all those next-to-the-skin garments that dared not speak their name.How the underwear culture has changed! On fashion runways, underwear is turning into outerwear as designers unveil the "lingerie look" for spring. And in newspapers and magazines, tiny triangles of silk and satin and peek-a-boo lace regularly show up in full-page advertisements, each more seductive than the last. Some of the most provocative ads appear in The New York Times, courtesy of Bloomingdale's. Lean, long-torsoed women, clad only in plunging bras and V-shaped bikini briefs, fill the page, drawing a reader's eye as much to their nearly naked bodies as to the wispy products they are selling. One reclining model arches her back and stretches her arms above her head as if in surrender. Another, her lips fixed in a pout and her hands clasped over her bare midriff, offers a languid, come-hither stare. Elsewhere, a three-page magazine ad for Natori lingerie begins with a naked female body. It ends with an underwear-clad model on her back, fingers outstretched to form an eerie shadow behind her. Scantily-clad lingerie models have been around for decades. Forty years ago, Maidenform created a sensation when it introduced the Maidenform Woman. Dressed only in underwear, she showed up everywhere in town, offering such memorable lines as "I dreamed I stopped traffic in my Maidenform bra." Although controversial at the time, the ads were at least selling underwear, not seduction. The models, active and energetic, projected a certain no-nonsense air as they carried briefcases or conducted business in financial offices and architects' drafting rooms. As the company proclaimed in one ad campaign, "The Maidenform Woman knows where she's going." The same cannot be said for the recumbent figures featured in ads today. Their passivity often borders on helplessness, with an occasional hint of bondage thrown in for good measure. What at first glance appears to be sexy, may - on closer inspection - simply be sexist. Jean Kilbourne, a media critic in Newton, Mass., who has spent 20 years studying images of women in advertising, observes that many of these ads "reinforce the stereotype that women are fundamentally sex objects and they're fundamentally passive. It's part of the cultural climate in which women are seen as objects and therefore are more likely to be abused. It's very easy to abuse a thing, an object, especially if she appears to be asking for it." Lingerie ads, Ms. Kilbourne says in an interview, can subtly place women in a no-win situation, pressuring them "to achieve a certain image" by having a perfect body and wearing lacy underthings. "If you don't achieve this image, [the implication is that] you've let yourself go - you're not attractive or desirable. But if you do achieve it, you're liable to be seen as seductive, a temptress - the devil incarnate, seducing men." "Temptress" is, in fact, one of the charges being leveled against the Palm Beach, Fla., woman who has accused William Kennedy Smith of rape. Because she was wearing Victoria's Secret lingerie the night of the alleged attack, his lawyers have sought to portray her as promiscuous and "asking for it." Judging a woman by the label in her underwear is cruel punishment in an age when lingerie manufacturers spend huge sums to give their gossamer garments respectability. Women have gained considerable physical freedom since the days when the whalebone stays on Grandmother's corset dug into her ribs and kept her from moving freely. By contrast, today's frilly featherweights offer the ultimate in comfort and mobility. Even so, Kilbourne says, "We're still very much bound. Today our corsets are internalized. We don't need to have ourselves laced up, because we're supposed to starve ourselves into this ideal figure. As horrible as corsets were, at least one could take them off eventually." The new chains - as thin as string and as stretchable as Spandex - have brought women no closer to emancipation. It is not the clothes women wear but the way they are looked at and look at themselves that will finally bring them freedom.