I don't think it's tacky at all to do it that way. My friend had to do it that way because her church was really really small and there was no way everyone would fit (and it was in June with no a/c) and it was the church she grew up in so there was no alternative to a bigger venue. So instead of possibly having people feel bad that they weren't invited at all, she just invited them to the reception, which I think most people would think is the best part anyways. The ceremony was maybe 15 min long.