I think the Carholic church has decided it's dangerous to accept parts of the Bible as metaphor, so they must force a literal interpretation on everything - even to the point of absurdity. Jesus said it's his body and blood so it must be. Because you know, everyone making a symbolic gesture always says that's what they're doing...

As a non-religious person, I wonder, if you reject a bunch of catholic ideas (the authority of the pope, the Eucharist, catholic social teaching, etc), why not just be protestant?

The Catholic Church is broadly accepting of metaphorical interpretation, but with respect to the blood and wine specifically they committed to that literal interpretation a very long time ago.