I have wondered for some time, and asked several religious and atheistic friends and associates this question on several occasions. so far, received no satisfactory answer.
Most people, upon hearing that someone they know see's no issue with the consumption of human flesh (not necessitating murder), will find the concept and often the individual in question, quite revolting.
However, I have two issues with this.

1. This position requires one to make the assumption that human life is more valuable than animal life, given the sheer number of people who consume the flesh of formerly conscious, intelligent creatures, capable of complex thought and emotion (at least when compared with other forms of life). An example would be the holocaust. We are all in agreement, no doubt, that the industrial murder of millions upon millions of non-combatants, who were not seen as human being by their captors and executioners, as one of the worst atrocities in history. Try, for a moment, to imagine how many animals have had their throats slit or have been scalded to death in boiling water (a process employed to remove hair and make the skin more supple) since 1945. Humour me for a moment, and try to imagine the figure. Billions? Tens of billions? Hundreds of billions? It's difficult to say, and even harder to imagine. How is this any less of a crime? Why is it ok to slaughter animals on a level that would embarrass Hitler and Mengele on a bad day, while eating the flesh of a human is not even a topic for discussion? Given that the slaughter of animals, at least in the western world, is no longer a necessity, given the abundance of alternatives. Is taking a life outside the bounds of necessity not murder?
2. The vast majority of us know a lot of religious people. And while we may disagree with them about their beliefs or their basis, we still tolerate the, befriend them, sometimes even have intimate relations with or marry them. Let's examine the book of Leviticus for a moment.
   “If a man lies with a male as with a woman, both of them have committed an abomination; they shall surely be put to death; their blood is upon them.” – Leviticus 20:13. “You may purchase male or female slaves from among the foreigners who live among you. You may also purchase the children of such resident foreigners, including those who have been born in your land. You may treat them as your property, passing them on to your children as a permanent inheritance. You may treat your slaves like this, but the people of Israel, your relatives, must never be treated this way.” – Leviticus 25:44-46. “Whosoever … hath any blemish, let him not approach to offer the bread of his God. For whatsoever man he be that hath a blemish, he shall not approach: a blind man, or a lame, or he that hath a flat nose, or any thing superfluous, Or a man that is brokenfooted, or brokenhanded, Or crookback, or a dwarf, or that hath a blemish in his eye, or be scurvy, or scabbed, or hath his stones broken … He shall not go in unto the vail, nor come nigh unto the altar, because he hath a blemish; that he profane not my sanctuaries.” – Leviticus 21:17-23. “When a woman has a discharge, if her discharge in her body is blood, she shall continue in her menstrual impurity for seven days; and whoever touches her shall be unclean until evening. Everything also on which she lies during her menstrual impurity shall be unclean, and everything on which she sits shall be unclean.” – Leviticus 15: 19-20. That's a small handful of injunctions from one book of the Bible. Has anyone noticed that we still regard people who take the Bible very seriously, as the pure word of god, as being perfectly decent human beings a lot of the time? What about Muslims? They see the Qur'an in the same way. The amputation of thieves, Qur’an 5:38-39, Sahih al-Bukhari 8:6789 & Sahih al-Muslim 3:4175-79. The rape of women, Muslim or non-Muslim, covered or uncovered, Qur’an 2:223, 4:3, 33:59, 23:5-6 & Sahih al-Bukhari 1:367 & 5:522. Forcing dhimmis to pay the jizya. Qur’an 9:29, Sahih al-Muslim 19:4294. FGM doesn't appear anywhere in the Qur'an. It does however appear in Ahmad Ibn Hanbal 5:75; Abu Dawud, Adab 167, Sunan Abu Dawud 41:5251 and Sahih Muslim 3:684.
   Perhaps "Why is cannibalism considered so bad?" is not quite specific enough. Why is cannibalism considered something that is beyond discussion, so vile is the concept, yet people who believe some of the aforementioned garbage that we can all agree is indescribably evil, are often considered perfectly decent human beings. This makes very little sense to me.