I think mainly because one of the two sets of parents is very religious and conservative... I don't know that they would not approve of things, but if bringing their son into that mix might bother them I feel some level of responsibility to give them some notice.
Though it's not as if things are toxic between W and I or anything like that. I imagine, assuming things are that day like they have been, most folks could spend the few hours with us and perhaps not even have a clue unless they look hard.
Married 6 together 8 Me:38 W:31 second marriage for both SS12, SD10, S6 Bomb: 9/8/11 (day before our 5 yr ann) W moved out: 2/18/12 D final: 11/12/12 Share S 50/50. Spend as much time as I can with SS & SD