Below are a couple of links to explanations of this charge against Catholics.
I have read about and heard the charge but have never actually had anyone say it directly to me. My first thought, is that we consume Our Lord body and blood, soul and divinity -- in the form of bread and wine -- whose substance is changed into the body and blood of Our Lord.
Anthropologically, speaking -- cannibalism is not uncommon in reality and is often used as a perjorative because while not uncommon it is associated with the primitive, wicked, etc. And, so by throwing that charge out there most people are hoping to shock and repulse people, keeping them from either learning more about the topic, or ending a conversation they think they have won, or just to be obnoxious without giving it any thought.
There are several conditions related to cannibalism which are not found in the Eucharist and the articles address them. I hope these help some.
And actually, there's a serious point to that. I could put it this way: "Let's admit, just for the sake of argument, that we can stretch the meaning of the term 'cannibalism' so that it can be made to cover the consuming of the Eucharist. We object to cannibalism as an evil and immoral (as well as unhealthy and disgusting) act. But as God himself has mandated that we eat his flesh and drink his blood, we have no legitimate authority for saying that this command is evil and immoral, since God is the very source from which we derive our sense of good and evil. To say "God's command is evil" is to utter an inherently absurd statement. So the 'cannibalism' involved in consuming the Eucharist must be good, not evil. All other forms of cannibalism are, we agree, evil. To use the same word to describe something that is good and other things that are evil is, again, absurd. (It's always absurd to use the same word to describe things which are different in their moral nature). We can only escape the absurdity by giving different names to things which are different in their moral nature. Therefore we can't, in fact, use the term 'cannibalism' to describe the consuming of the Eucharist".
Since I make no claims to philosophical sophistication, I invite others to point out any flaws in this argument. It seems sound to me.
brilliant! That is why it will go right over his head.