Women are under-represented in physics, math and computer science. Intersectional analyses show why: Because the norms of typical departments fit the cultural skills of affluent White male students. This project, an ethnography of a liberal arts college with lots of women in these majors, is grounded in the activist roots of intersectionality. The cultural norms of these departments are very different from what other ethnographers of university STEM have found. They include: work hard and practice; work together; be friendly. Professors actively establish these norms and also take feminist, anti-racist public stances. The author uses intersectionality not only to show why these departments are good places for women but to point to actions STEM faculty can take.