It's not inherently wrong, from an ethical standpoint. It's totally neutral. KILLING another human being in order to eat them is ethically wrong, but eating the meat (or anything else) has no inherent ethnical value. The only argument that holds any weight is that you are eating a being that is as fully sentient as yourself. Then again, when they are dead, putting aside religious sensibilites, they are just meat. And tastey meat, supposedly.
The argument against human beings consuming other humans is no stronger than against that of eating meat in general. I choose to eat meat on occasion, but I'll admit... Masticating on the flesh of a living being that felt and lived and desired seems barbaric to me.
Many cultures around the world have practiced some form of cannablism. In a ritual context (before the advent of colonialism), it was not taboo. Eating another human being solely for survival is not generally taboo even in our culture, though there is still stigma attatched to it. It all comes down to cultural taboos. You'll be stigmatized by your society if you do eat another human being, but there is nothing inherently "wrong" with it so long as you don't hunt another human being. Many Jains and Hindus, I'm sure, find it reprehensible to eat the flesh of any living being. It just depends on the culture and time period in which you were born.
This is an anthropological perspective though. Generally, people around the world don't think it's -horribly- wrong until some Christian missionary tells them it is. They may have some taboos regarding the way they are consumed, what is consumed, or in what context it is allowable though. Generally, people are too sentimental to wantonly eat their own kind, unless you're a sociopath and don't really feel anything at all; you just fake it so society accepts you.
~Ethno