Yes, that is extremely rude. This is a HUGE pet peeve of mine. Here in the USA it's called a tiered wedding and it's one of the rudest things you can do in terms of inviting guests. The exception is if you have a truly private ceremony (example, just you and your OH and both of your parents and closest family, less than a dozen or so people) and then have a large reception afterwards.
I was once invited to a tiered wedding, it was a little different but I was invited to the reception only. I figured the ceremony was private, so it was fine...until I got to the reception and there were 150 people there, and I found out from chatting to people that almost everyone in the room had been invited to the ceremony as well. There were only a handful of people out of 150, myself included, that didn't make the cut to be invited to the ceremony. I got really embarrassed and hurt, and split right after dinner. I wasn't going to hang around knowing that I wasn't important enough to come to the ceremony, but that I could certainly fill a seat at their reception because they needed a certain number of guests.
End rant.