S mart, spirited, and happily pregnant, Kristine Kupka told friends she was going to check out an apartment with the father of her baby. That was ten weeks ago. She hasn't been seen since.   She was a romantic -- you could see it in her room: handmade muslin drapes looped through dogwood boughs, low tables bearing plants, vases, goblets. A shawl-draped dresser displaying a tin Barbie box, pictures of herself as a child in Wisconsin and of her sister Kathy's adorable 2-year-old son, named Marshall, after Thurgood Marshall. The books on her shelves (Voices of Freedom: An Oral History of the Civil Rights Movement, The Philosophy of Right and Wrong, Development and Dependency) advertised a feisty, cerebral idealism. And on the floor near a stack of CDs (Jamiroquai, Liz Phair, Jeff Buckley, the Fugees) were more books, including a dog-eared copy of Two of Us Make a World: The Single Mother's Guide to Pregnancy, Childbirth and the First Year. She was five months pregnant; and though the baby was unplanned and she was adamantly pro-choice (after vegetarianism, reproductive freedom was the cause she fought for most fiercely), she never considered abortion.