As one who is on the receiving end of these demands for the convenience of the new family - it is a pain. I have a life and when it is impacted and I am made to feel unreasonable for not acquiesing then it is not acceptable.
Demands? A pain? To be responsible for your own DCs?
I've never understood the mentality that when a child is not with a parent (either at school, paid childcare or their other parent) that parental responsibility for the child is temporarily suspended and therefore, changes to those plans are resented.
They are your DCs, and you are responsible for them all the time.
As I've said, I'd rather they were with me, so I could help them deal with there own conflicting emotions after their Dads wedding, than their Dad and SM have to worry about childcare for my child on what is, for them, a special day - whether or not they choose to consummate the marriage or not!
Given that a wedding, it's not like it would mean I'd be dropping plans with no notice, it's planned in advance.
And for my child, I would (and have) cut short days out, cancelled appointments and generally made sure I was there when she needs me, even if she was scheduled to be in her dads care at the time. Isn't that what parents do?
(As an aside, neither of my wedding nights could be described as "shag fests", but each to their own.)