Kristen Stewart says she felt a “huge responsibility” to define her sexuality after finding fame in the “Twilight” movie franchise. She says she worried about “forsaking a side” by not formally coming out as lesbian or bisexual. But she’s glad today’s young stars don’t have to. The 29-year-old actress, starring in the movie “J.T. LeRoy,” sees a shift in culture that’s allowed young people — in and outside of Hollywood — to accept fluidity in gender and sexuality.