My comment will be quick as I have to get the kids up and to school and then won't be back till after sports tonight.
Just wanted to explain that Catholics aren't being "meanies" because other Christians are not invited to Holy Communion. It is because we fundamentally believe different things about what is happening at this time during the Mass. If you read chapter 6 of John this is what our faith and belief hinges upon. Jesus explains again and again that his followers will have to eat his REAL flesh and drink his REAL blood:
"For my flesh is real food and my blood is real drink. Whoever eats my flesh and drinks my blood lives in me and I live in that person." John 6: 55-56
This causes consternation amongst the Jews and among Jesus' own disciples. "After hearing it, many of his followers said, 'This is intolerable language. How could anyone accept it?" John 6:60
"After this, many of his disciples went away and accompanied him no more." John 6:66
This is the only time in the Gospels when disciples leave Jesus because of doctrine. This is also the time when scripture mentions that Jesus says Judas is turning away from him.
For the first 1500 years of Christian history (so 3/4 of the history of the church) all Christians believed this. If they did not, church councils and teachers going all the way back to the apostles made it clear that it was heresy not to believe this fundamental article of faith. Since the Reformation some Christians have chosen not to believe that the bread and wine became the literal Body and Blood of Christ. Catholics continue to believe (along with the Orthodox and other ancient churches such as the Copts) that during the Mass the bread and wine become Jesus and that's why we call it the Real Presence.
If you go into a Catholic church you will always see a lamp lit next to the Tabernacle (except on Good Friday when the Tabernacle is empty). This is to show that Jesus is present in what was the bread and wine and is now reserved in the Tabernacle. This explains why there is a sacred and reverent atmosphere in Catholic churches. This is why Catholics kneel and prostrate themselves during liturgy and prayer because we believe Jesus is truly present, not just spiritually.
For this reason First Holy Communion is a Sacrament in the church which those who receive for the first time must be carefully prepared for so they understand exactly what we believe is happening. To come and receive without believing is therefore considered sacrilegious to Catholics. There may be Catholic commentators on here that do not believe this, but it is fundamental of being Catholic and something Catholics have been martyred for through the ages.
I am not trying to convince anyone of the truth of this doctrine - all I want to explain is what Catholics (and Orthodox and others) believe.