THE trouble often begins with a woman's initial visit to a doctor. The doctor asks, "Are you sexually active? The doctor, looking shocked, proceeds to lecture the woman on the importance of preventing unintended pregnancy until the patient reveals that she is a lesbian. What may seem like little more than an awkward moment is typical of the ways in which many doctors alienate lesbian patients and may ultimately jeopardize their health, a recent study concluded. Because of insensitive treatment as well as flagrant prejudice from doctors, lesbians have fewer checkups than they should, fail to get important screening tests like Pap smears, mammograms and cholesterol tests, and are therefore less likely than other women to have cancer and heart disease diagnosed in the early, most treatable stages, says Dr. Katherine A.