Sorry this is so long!!
One of my friends is extremely artistic - he paints, does photography, sculpts, etc. When he heard that I'm getting married, he decided that for a wedding gift, he wanted to do the photography at our wedding for us. I told him we'd already contracted a photographer, and he said he wanted to do a wedding album for us (for free) in addition to what our photographer was going to do.
It was a really sweet thought, but...
1) He kind of invited himself and wasn't someone I was originally planning on inviting at all.
2) FI and I chose our photographer for a reason, and I really don't want to push the boundary of the "only photographer" clause in the contract.
3) (Most importantly) FI isn't comfortable with this guy being at the wedding, and I totally respect his feelings on this.
I've been EXTREMELY noncomittal when my friend has brought up the wedding, because I just honestly don't know how to come out and say, "We appreciate your offer, but have to turn you down" without sounding like a complete jerk. Not only are we turning down his wedding gift (not that I EVER expected one), but we're telling him he's not invited to the wedding.
I guess I just feel really terrible because it's not that I don't like his work, it's just that the situation isn't one that's comfortable for FI and I, regardless of how well-intentioned.