Ok I'm going to explain this in terms of person A and person B.
Generally on weeknights person A cooks and person B washes up. The last two nights person B has been busy so person A has both cooked and washed up (which is fine).
Last night person A decided to wash up whilst cooking tea (so all the mugs/plates/bowls etc used since the night before) as tea was only shock horror chip butties, so only one baking tray and 2 plates were being used.
Therefore after tea person A didn't wash up again as thought it was a waste of water to run a fresh bowl.
This morning person B has noted that there are 2 plates, a baking tray, and 2 mugs that haven't been washed. Person A explained there was no point running a fresh bowl of water.
Person B is not happy and said that as they are chief washer upper it should be done how they like it. Person A has said that if they are that bothered they are happy for person B to leave 5 items of things to wash up tonight and they will do them, to which B replied they won't be doing the washing up at all (even though person A is busy tonight and will be rushed enough cooking tea).
So WIBU unreasonable A or B?