I disagree with Transubstantiation, but don't find it practically offensive. At least, it encourages faith in the possibility of a mystical connection with the living Christ during the Eucharist. What I object to is the exclusion of Protestants from partaking of the holy elements. Here are 2 negative experiences that demonstrate why:
(1) I once regularly attended a joint Catholic Episcopal charismatic prayer meeting, which culminated in an ecumenical Communion service. When the local Catholic bishop got wind of this, he prohibited Catholics from taken part; they had to confine themselves to their own Eucharist. The result? A bitter dissolution of the long-standing loving prayer meetings!
(2) When I was a Theology professor, a monk invited me to take part in a Catholic Communion service. I jumped at the chance, but when the other Catholics saw me participate, they were outraged and I stopped participating. I was most angry because at the time I was unaware that I was forbidden to partake of Catholic Communion and was only accepting the Catholic monk's invitation.
Just to say so, I used to always find it offensive as well that Catholic churches wouldn't allow me to take communion. But the older I got, and the more knowledgeable in the scriptures I got, the more I realized how absolutely appropriate it is.
The one factor that Protestants don't take into account is that Catholics don't let Catholics take communion either. Not until they have their Confirmation. Confirmation leads to your First Communion.
Whether or not this is the reason, the fact is, the leader(s) in any local church establishment has a responsibility to those in his (or her) congregation. Paul says that if you partake unworthily, you eat and drink yourself to damnation (1 Corinthians 11:26-29). So it is important as a pastor or priest to ensure that you are not leading people to condemnation by allowing them to participate in communion if they are non-believers, apostate, heretics, etc.
Through the process of Confirmation, the Catholic church assures that the participant has been duly educated in what the Catholic church believes to be sound doctrine. If the individual is unworthy in their motives or intentions outside of that, it is between that person and God. But the church has done its duty.
So when you encounter a Protestant, most of whom have absolutely no real education in their own faith, it's actually a very
responsible thing that the Catholic church refuses to allow people to participate whom they cannot confirm have been educated in sound doctrine and are professing a true faith.
Far from having a problem with the Catholic church for denying me communion on the rare occasion when I attend that sort of church, I actually find it more offensive that the Protestant denominations I have attended give out communion to people willy-nilly, with no idea whatsoever what the faith, profession, and spiritual condition is of the people partaking. It's very irresponsible. They don't even bother to give a brief message beforehand to explain what communion is, what belief it represents, the humility with which you should participate, etc.