Lets say that person x is going out with person y...
And things are going quite well, but a little tricky because it is a long distance relationship for the most part.
And then (after about 9 months) person y says 'I don't want to presume anything, but I think I should let you know that I've been on a few dates recently'.
Lets suppose that there wasn't an understanding about not dating other people, but that neither x and y had done that for a time while they were trying to make things work.

Lets suppose that person x says 'well, either we can work on developing and maintaining our relationship or you can date other people - but not both'.

Long distance relationships ARE hard, but lets say that x and y could actually spend one year together if only y was prepared to wait another 6 months...

Does it sound reasonable for x to say what x did? Or does it sound like an unfair ultimatum?