In women, the prolonged starvation often 'shuts down' their menstrual cycle because their bodies don't have the caloric budget for that shit anymore. And if you can't menstruate, then you cannot become pregnant in many cases, but not all; it's possible, but rare, and can likely result in a miscarriage. And if you can't get pregnant, then why does your body need you to feel sexually excited? In terms of biological necessity, it simply doesn't anymore, and your body kind of needs those spared calories to keep your brain, muscles, and other organs functioning, if barely. So I would imagine it's rare that anorexics have a libido, unless they're using drugs or something.