Dating for a couple of
They don't want to spend forever with that person, but they don't want to see that person happy with anyone else either.
For instance, a divorcee who is getting back into the dating game is advised to keep things light for a while, so as not to try to avoid dealing with hurt feelings by jumping into another serious commitment right away.
They're just out for a good time and to have fun with each other - that's the extent of the relationship.
A couple can go on like this for a while, or the relationship between them can turn more serious, sometimes without their intention or expectation.
Just because the relationship is casual does not mean that you can't be honest with that person about your feelings at any point in the relationship.
Honesty does not equal seriousness if that isn't what you want.