The moral standing of a killing is completely unrelated to consent for most people. Most would say murder doesn't become moral because the victim was suicidal. The killing itself is the basis for the moral stance, not the desires of the victim. The act is valued. Consent is neutral.

Sexual interaction is different. It does not necessitate a state change. It can be done to positive effect if done with consent. But animals, much like children, are viewed as insufficiently capable of granting consent. The consent is the basis for the moral stance, not the act itself. Consent is valued. The act is neutral.