The Demands of the Flesh by March Hastings

Ellen, a young widow suffering from physical frustration goes through a period of promiscuity involving several men and a brief affair with a lesbian, Nita. Oddly enough for this sort of borderline-risque stuff, the lesbian character is well and realistically drawn; realizing that Ellen is basically normal, she helps keep her on an even keel until she remarries. ~ Marion Zimmer Bradley