True, I did avoid the topic of the Real Presence, and I’m not sure either how it applies to the Catholic doctrine on sacraments. However, I’ll take a stab at it. Since the Catholic idea of sacraments is that they are not merely symbolic, but actually do what they symbolize, then it also follows that the bread and wine are not only symbols, but actually are what Jesus said they were, His body and blood.
I have trouble with the idea that Jesus was speaking metaphorically about the bread and wine and body and blood, because they don’t make sense as a metaphor. A metaphor to be meaningful as a metaphor has to make sense, to compare with something in a meaningful way. If Jesus said, I am the Way, or I am the door to life, or, my words are the way to life, these are sayings that could be understood as metaphors.
But to say His flesh and His blood? How do they come in? What do they have to do with it? How can His flesh and blood be a metaphor, or even a symbol, for anything? None I can think of. Why would Jesus use a metaphor so repugnant to Jews such as drinking blood, and eating human flesh? I can’t think that He would, therefore when Jesus said, this is my body, and this is my blood, the “is” is not metaphoric but literal.
Paul says we are part of His body, and so we literally, physically, become part of His body by partaking of the eucharistic sacrament.