Fact: Lainie Kazan is Jewish. Fact: Lainie Kazan is sexy. The two, according to the singer whose vocal style is positively aphrodisiac, are not unconnected. "A certain eroticism is inherent in the Jewess," says Lainie. "There have been many Jewish sex symbols, but we've forgotten about them." She cites such examples as Esther--the Biblical Jewish maiden who became queen to a Persian king and later saved her people from destruction by using her abundant charms to gain his favor--and Delilah, whose hair-razing exploits with Samson are well known. On these pages, Lainie offers a graphic contemporary illustration of her points. But Miss Kazan's cantilevered configuration doesn't tell the whole story. She's also an internationally acclaimed singing star--and she grooves on it. Lainie tackles every song as if it might be her last. When her sensual delivery becomes too intense for singing, she shifts into breathless speech. As far as Lainie is concerned, however, one of her career problems has been that she's too good at singing. Since her Brooklyn childhood, she has wanted to act, a desire partially fulfilled when she played the lead in Funny Girl. She has since appeared in several films but didn't land her first big dramatic starring role till recently. "Actually, I started in this business as an actress and got into singing later," she says, "but most producers think of me only as a vocalist." Lainie makes use of both talents during her night-club performances. She combines a big, sultry mezzo-soprano with some effective acting that turns on her audiences while setting her apart from them. One critic, after viewing Lainie's expressive delivery, said he felt like a Peeping Tom. "My biggest self-criticism," Lainie says, "has been that I try to be everything--a little girl, a woman, sad, funny--all in one song. I'm learning that you can't be naked onstage. People get confused by seeing all of you at once. But I want to move people, whether it be for good or for bad. I just don't want them to be indifferent." Considering her bountiful assets--visible and audible--that's not very likely.