Why do Catholics "exclude" their separated brothers and sisters belonging to the Protestant denominations? Why can't a Protestant receive Holy Communion at a Catholic church?
First, let's remember that the road goes both ways.
For example, the Westminster Confession of Faith (held by the Presbyterian Church in America and the Orthodox Presbyterian Church) calls Catholics "idolaters." The reason for this, no doubt, is that Catholics worship the Eucharist as the Lord Jesus Christ Himself. If the bread is still substantially bread, then this qualifies as idolatry and we are in fact idolaters. This is why Calvin called Catholics idolaters and it also why nearly every PCA session in America would exclude a practicing Catholic from the Lord’s table because the Catholic holds to this so-called “idolatrous” position. The PCA's conviction is actually one of charity, because someone shouldn’t receive the PCAs communion if he is an idolater. No one disagrees with this, we just disagree over what constitutes an "idolater".
The Catholic position even more so. In most cases, Protestants:
A) deny the the substantial change of bread and wine into the true Body and Blood of Christ;Consequently, its impossible to “include” Protestants since they would “protest” elements of the Holy Mass nearly every minute of the liturgy.
B) They don’t “acknowledge the body of the Lord (either substantially or ecclesiastically as described in 1 Corinthians 11);
C) They deny the sacrificial aspect of the Mass;
D) They deny the liturgy of the Mass which does invoke saints at times;
and E) They reject the papacy and the apostolic succession of the local bishop–both are commemorated in the Mass.
So Protestants are welcome to enjoy Eucharistic communionem in sacris if they would like to do so–but only if they truly believe what the Holy Sacrifice of the Mass is and what the liturgy says. If this were the case, they would be interested in becoming fully Catholic. They would cease "protesting" and would cease being "Protestant."
Likewise, if I desired to receive from the "Lord's table" at a Protestant church, I would have to renounce several of my beliefs (e.g. abjure transubstantiation) and hold their doctrines (eg. justification by faith alone, etc.).
In conclusion, the Catholic Church isn't doing something mean or intolerant. The Catholic Church is merely practicing charity. For us, the Eucharist is the source and summit of our lives because It is Christ Jesus. It would be wrong for us to allow others to partake if they denied this reality. Similarly, it would be wrong for a Protestant to allow a Catholic whom the Protestant deemed "idolatrous" to receive Communion.
It is a sad state of affairs that brothers and sisters who call upon the same God as Father and seek the same Jesus as their Savior to be so deeply divided. Let us pray to the Holy Spirit to bring us back together again into one Church, united in the Apostles' teaching.