I agree with changedname, part of being a blended family is sometimes being responsible for all the kids even the ones which aren't biologically yours, because you willingly entered into a relationship with someone who has children.
Now personally I wouldn't expect my DP to have my ds over a weekend if I were going off on a jolly on my own, but I know that he would, and in fact my DS stayed here with my DP when I was in hospital towards the end of last year, and if he started telling me that he wasn't prepared to look after my child and that his father should be doing it he would see the other side of the door very quickly.
Equally, while I would expect my DS to stay with me in the event that eXH were going away during time he was meant to be staying there, if eXH decided to agree with his DP to have DS instead that arrangement would be between them, so the posters saying that the OP should get on the phone to the ex and demand she have the kids are way out of line.
This arrangement is between the OP and her partner, it's not the ex's responsibility if the partner has decided to go away on his contact weekend. And if the OP is that resentful then likely the children are aware of it - kids aren't stupid. If they complained about OP to their mum the wording might have been as simplistic as that she doesn't do washing or cook what they want, but it's probably far more complicated than that in their heads. For them, they're living with a woman who resents their presence, doesn't really get on with their father, and sees them as an inconvenience.
As for the partner, well if the OP isn't happy with him then she should leave. It's that simple really.