We are getting off track here, but as someone who shot a few hundred weddings in a previous life, the fact of another camera intruding on the wedding coverage is very annoying. I don't want to get into a discussion of your rights to shoot at a friends wedding but the following is a few reasons that it is distracting.
1. It is my living, and you giving or selling prints cuts into it.
2. I want the B&G and family to pay attention to me. If they are looking around at other photographers when I'm posing or shooting, the picture quality suffers considerably.
3. At the church amateurs will step in front of me while I'm trying to shoot. Shots that can't be redone.
4.Time is precious and if they are posing for you, you are stealing valuable time.
5. The B&G saw my work previously and that is why I was hired. They already know the prices and the quality. An amateur getting in the way and undercutting is hurting my business.
I'm sure that whatever you occupation is, you would not be impressed with someone coming in and hurting your work product or disrupting your work.
Anyway, just another point of view.
Thankfully I paid my penance and have not shot weddings for years.