I think you made the best choice.
The 12 year old will have a great time at a sleep-over.
Kudos to you for not getting mad about the "no-kids" edict and just bringing him anyway. I have a million horror stories of less polite guests than you!
Personally, I think 12 is old enough to handle it if he feels "left out". It's a part of growing up. There will be things he can't do or places he can't go. He can't smoke or drink or drive a car either! Any kid whose parents have been left with the babysitter on date night have had the same "left out" situation. It wouldn't be a problem for me. I'm sure many people would feel differently though.
Seriously?! Maybe I'm just uber polite but there is no way I could do that! Yay, my parents brung me up proper like innit!
I think the big problem part for my hubby is that the twelve year old is my stepson, so whereas I'm getting to take my one and only child, he has to deal with leaving one of his kids home while he and his wife go away with the shiny new "their" baby.
I can sympathise, to a point, but at the end of the day it's up to the bride and groom, and I can totally understand people having kid free weddings! We had kids at ours, but basically only because we felt we couldn't have SS and none other... lol! Besides, the kids distracted each other mostly so the adults could chill, and most of us were staying at the reception venue so it wasn't like someone was "designated driver"!