i think 'I will go ....." reads better than "I will come....." . "I will come to school, I will come to the meeting, i will come to the shop..... sounds ????????
Agreed at least it shouldn't be marked WRONG, which it is.
Since it's "their" wedding and the speaker isn't talking to "them", I'd presume the speaker regards the event as being removed enough to say "go" rather than "come."