I find it funny how 200 years ago we wouldn’t have been having this discussion. (Well, of course not, we wouldn’t have been alive.) What I mean, though, is, 200 years ago, there was never any question about open communion. It simply was never done by anyone - Protestant or Catholic - until recently, and for a simple reason: if you did not believe what the church distributing the bread and wine believed, you didn’t partake in their communion. To be in communion is to believe in one accord. And on many things, most of all the Eucharist, a Calvinist is in disagreement with a Catholic. And that used to be taken quite seriously.
Perhaps this is what happens when people forget that the creeds they recite and beliefs they hold are claims of truth (rightly or wrongly). And if that bread and wine is claimed to be the Body and Blood of Christ, but you don’t believe such a thing to be possible, why would you want to partake in that which you don’t believe, until you do believe?