DangerMouth, I don't think it's a question of how much you like the happy couple.
For me, I missed my best friend's wedding because she invited me on a trip, I chose to go somewhere else, and only found out afterwards that she was eloping. I was very, very upset with myself for missing her wedding; it doesn't matter that I had no way of knowing that was what I would miss. The end result is still that I missed her wedding.
For many others, there are weddings I would not want to go to. I have a girlfriend who is dating a semi-abusive man. I would not support their marriage, and I would not attend their wedding.
I have family obligations that require me to go to events for an hour or two, but I don't want to go for the whole day.
Especially with a barbecue, I can see myself hearing that it started at 4, deciding to show up at 6 because it's a come-when-you-can type of event, and then arriving either during or right after the surprise ceremony.