I'd always gotten the impression that the bishop wasn't there in an official capacity, as such. He was there as a 'friend' of Lex. The point being to illustrate how much influence and power he had at the highest political levels. A kind of "Look how powerful Lex Is. He can even get the bishop to do his wedding, even though we're not in a church but having a private ceremony." It was a typical Lex attempt to impress.
So for those reasons, it didn't seem odd to me at the time.
LabRat