[NSFW] When you start, do you feel an obligation to continue until it's over, no matter what?

In another question, someone mentioned being bored during sex. It sounded like she just lay there and thought of England after that (i.e., just did her duty). It made me wonder whether people feel an obligation to continue even though they are not having a good time. If so, why?

Does it depend on the relationship you have with the person?

Does anyone just stop and tell their partner it’s not happening, so it’s not going to happen?

What do you do when it isn’t any good?

What circumstances would make you stop right then and there?