Grow-up! If you don't want to say "under God" in the pledge, don't. If you don't want to participate in a prayer, don't. Problem solved.
The problem isn't "being offended," or being forced to take part in a prayer you don't believe in. The problem is that something like "under god," or an opening prayer, legitimizes the assumption that religion has a place in the government.
It's less of an issue if you're in Congress, and they fly in an imam one week, a Jesuit the next, and a Baptist after that.
But if you live in a relatively small town? Where everything is closed on Sunday morning because 80% of the population is at church? And then you have a town hall meeting, and you want to open with a prayer because, hey, we're all God-fearing, good Christians in this town, right? So you invite the local pastor to open with a prayer, and then get to business about zoning regulations or whatever, with no mention of religion afterward.
Which sounds fine, until a few months/years/decades down the road when it's time to vote about gay marriage, or allowing teenagers to dance, or whatever.
I'm not "offended," and I don't think I'm going to be held down by guards and be forced to pray. But stuff like this contributes to the idea of "Well, America has separation of church and state, but we're all Christians, right?"