Actually I don't think it does, at least depending on where you are, and the definition for sexual assault might be different, but in my state (US) you can be charged with assault even for intent to attack someone.
Yeah I think its weird that it's considered more morally sound to make them waste away in agony then let them willingly end their suffering through controlled means.
Like, if they're gonna do it, they're gonna do it. Wouldn't it be better to make sure they do it in the cleanest way possible?
Alzheimer/Dementia is one of those few situations where I really can't blame someone for going out on their own terms. The idea of being trapped inside your own effectively disintegrating mind is terrifying.