I'd rather not debate this topic, but let me leave you with this little anecdote: At the dinner following my wedding in my husband's Presbyterian church, my mother-in-law took it upon herself to ask a family friend to say grace before the meal. He said a wonderful (and very Protestant) prayer. I stole a glance at my father - who had paid for the meal - and he didn't look too happy. He waited patiently for the prayer to finish, cleared his throat, and then said, "And now for the Catholic prayer," made the sign of the cross, and my side of the family launched into "Bless us, oh Lord..."
So, you see, in my experience, and that's only one of them, no one agrees on these things. All I know is that the person paying for the event should have some say in it. And that's all I'm going to say.
I'll give it a ping and maybe you'll receive other responses.
And you know too there is just a difference in what people are used to. There are different religious sensibilities. Even between Christians there are different styles of worship and different ways of experiencing. For example John Hagee...ugh! I can't stand that kind of preaching! I am just not used to it. He's a fine fellow but I can't listen to him for two minutes. By the same token a person who likes Hagee might think a Catholic homily which is usually more subdued and conversational is as dull as dishwater. So when you get people who don't care for your "style" and push it on them when they don't expect it like at a graduation ceremony at a public school it makes them irritated. People don't like to admit it but religious expression has a strong element of style or art to it and some people can't stand a certain style even though they may agree with what is being said. It's just another layer of why people get upset over who gets to say what, when, where and how about their religious convictions.