If both parties consent, it's wholly different than murder. If one party does not consent, their rights have been violated and the government is entitled to step in. That's the same for hetero- and homosexual behavior.
Can the mentally ill consent?
Duh, almost forgot. If both parties do consent, does that mean that it is no longer murder? Nope, it's still a murder. Go ask Kevorkian if you don't believe me.
Then of course you don't mind incest, bestiality and pedophilia, right?