The general belief is that the "accidents" form of bread remains while the substance of the body of Christ remains. Catholics receive Christ in the form of bread. So Catholics eat the form of bread that in substance is the body of Christ.

If you still need to call this cannibalism then go ahead if you feel like it scores you points. Then my answer for you is that Christ does what we have come to expect--takes the worst of humanity and turns into something good. The crucifixion itself for example. In the eucharist, the consuming of the body and blood of Christ does not diminish the being being consumed but in fact becomes a central act of giving life to all.