Why do Protestants not believe John 6 when it says Jesus' flesh is real food and that His blood is real drink?

In Matthew 26, Mark 14, and Luke 22, Jesus says of the bread, “This is my body.” He says of the wine, “This is my blood.” Rather than saying, “this is symbolic of,” or “this represents,” He says, “this IS.” In John 6, He repeats Himself, like He does nowhere else in Scripture, to emphasize the fact that He expects us to eat His flesh and drink His blood and that His flesh is real food and that His blood is real drink. There are several facts that point to the literal meaning that Christ meant to convey here. Fact #1: The Jews took him literally in verse 52. Fact #2: His disciples took him literally in verse 60. Fact #3: the Apostles took him literally in verses 67-69. If everyone who heard him speak at the time took Him literally, then all of us today, 2000 years after the fact, are also called to take Him literally, in accordance with the Scriptures. Also, in verse 51, Jesus says that the bread which He will give for the life of the world is His flesh. When did He give His flesh for the life of the world? On the Cross. We know that Jesus was not speaking symbolically here. Since we conclude that Jesus was speaking literally of dying on the Cross, we should also conclude that He meant what He said about eating His flesh and drinking His blood. 
 
When we believe that Jesus is talking symbolically here in John 6, we come across a real problem when it comes to John 6:51. Did Jesus give His real flesh and blood for the life of the world, or was it only His symbolic flesh and blood?