It’s a lot like engagement and marriage. You can’t commune

with your fiance until you enter into a public, sacramental, formally-binding relationship before God. Though you may feel like doing it, though you may appreciate the goodness of it, and though you may feel like waiting for the vows of marriage is a mere formality, either of you are free to walk away from that relationship up until the time the vows are exchanged (ever heard of a guy being left at the altar? It happens, and they ain’t married).
Same with communion. A Protestant may appreciate the teaching of the Real Presence, he may desire Communion, he may feel like waiting for formal initiation is a “technicality”, but until he makes those promises of fidelity to the Church, he should not participate in that which both signifies AND effects Catholic unity: the Eucharist.
Why are premarital sex and non-Catholics receiving Catholic Communion wrong? In a nutshell, it’s a lie.
Rich
P.S. For those that take Protestants to Mass with them and their friends don’t believe in the Real Presence at all, you can add that you are simply protecting them. As Paul says in his letter to hte Corinthians that to receive the elements without recognizing the Body and Blood of the Lord is to drink judgement on oneself. If they don’t recognize that this is truly Jesus Christ (and a devout protestant won’t), then you don’t want them to receive a harsh judgement from their lord.