Now admittedly this has, as others have experienced, become less of an issue as the stepkids have got older and my partner did always use to ask me ..... but .......
..... there was only ever one 'right' answer to being asked about a last minute change of plans, so TBH, I felt it was all lip service anyway.
Yes ... it does depend on the reason, but I hated feeling that our plans came second best to his ex's, and unless they were ever ticketed plans, DP's attitude was always that they could be postponed. To me, it wasn't quite so simple - ticketed or not, plans are generally looked forward to, and/or actually needed (e.g. if you'd been working extra hours and needed some down time to relax) - and then of course there's the additional issue of cost ... a couple of extra kids' meals wouldn't break the bank, but add in £70 of petrol, put 'entertainment' and it's not quite so simple. I'd also have to re-jig meal plans very often and/or buy extra stuff they'd eat as they're quite fussy.
Added to which - while I'd get a load of sulks and possibly a row if I dared to say that actually I'd been hoping for some chill out time and could he perhaps say no 'this time', DP had no such hesitation if he, specifically, had plans which would be disrupted by an unexpected visit - e.g. if he had something with 'the lads' planned. Then he'd have no problem saying no even if his ex did whinge and call him a crap dad etc. I used to remind him of this on the comparatively rare occasions I objected to last minute additional contact which didn't go down well but I won't have hypocrisy going on and accusations of 'not wanting' his children when it was different for him.
Pickles - what does your OH say when you reminded him you had plans and asked why he presented you with a fait accompli ?
Oh - and if you can, go out without him anyway (if you want obviously) - do something nice for yourself even if it wasn't what
you'd hoped for originally. Don't have him ruin the weekend and expect you to play housekeeper too.