I've always assumed it had more to od with taboo than anything else, but your question led me to ponder about alternatives. I'm not sure I buy freemo's theory about arousal, for I've read about the existence of arousal out of other forms of violence too, though I don't know much about its prevalence. one less cynical theory I came up was that victims of murder don't survive with trauma, whereas victims of sexual abuse do, and since sexual abuse is so common (sadly), depicting it could trigger a large fraction of the potential audience, so it would be ill advised. but that doesn't quite explain boycotts, does it? maybe the boycotts arise out of an unexpected alignment between victims, who'd rather not relive the trauma, and abusers, who'd rather not have their own crimes exposed. if this is right, it's really sad, because education, awareness and empathy seem to be the best defenses and preventions against such abuses.