On the level that you're describing, however, is this really all that different from being a military spouse? Perhaps joining the National Guard, knowing that there's a risk that you will have to leave your family and go join a war zone?
Yes, soldiers deploy with the hope of returning, whereas Mars One would be a certain one-way trip. However, unlike a military deployment, Mars One is so dubious (to be charitable about it)... it's questionable to say that anyone involved has REALLY committed to anything yet.
However, you're right. Regardless of how low the chances are that one will actually go to Mars, putting themselves forward like this creates a 100% chance of informing spouses that they WOULD be willing to abandon them. Still, there seem to be many earth-bound scenarios in which couples face this sort of thing anyway.