I voted YABU because there are so many different motivations of the IDMer. A genuine, I don't mind, have no agenda and am flexible to fit with what you suggest is not necessarily being unreasonable. A IDMer who does actually have a preference and will either undo a lengthy negotiation or gripe about it afterwards is unreasonable.
I was bought up with a passive role in my family. One major relation was of the opinion that children should fit around the adults. Another major relation was very opinionated with a short temper so it was easier to just oblige (although she didn't appreciate a IDM so you were doomed either way). It wasn't a good training for functioning as an adult.
I am easily pleased so do naturally stand back to let people with stronger opinions slug it out. So on a group meal on holiday where the vegetarian, the spice hater, the spendthrift and the people who don't want pizza/ Chinese again are squabbling over it, I genuinely don't mind as long as I get chance to eat before all the restaurants shut! Throwing another opinion into the mix really isn't helpful at that stage.
1:1, offering closed choices is better than an open gulf of possibilities which can overwhelm some people. I have got better at narrowing down the plan from either side of the discussion over time. I'm much less likely to IDM and will start with my caveats (I'm free all weekends that month except...) rather than IDMing and then undoing it at the end (which would be unreasonable!)
What is really needed is some honesty on either side. If person A really wants to see the Lion King over Fast and Furious say so and present it as an invitation to see it rather than a choice. Only offer what works for you and don't keep opening up more and more choices. If you want the IDMer to decide for once tell them.
If an IDMer is infuriating and not likely to be malleable, adjusting your approach to forming a plan to avoid the IDM is the part you can control in the interraction. "I'm making a cup of tea, would you like tea too?" will either get an agreement for tea or a counter argument for coffee rather than giving space for an IDM. Ultimately, it's only our own actions we can control.