a: No one should force their morality on anyone else.

b: That sounds like a moral principle.

a: Yes.

b: Would you like to force that on others?

a: Force what?

b: Your moral principle that no one should force their morality on anyone else.

a: Yes, no one should do that!

b: Then I guess you do believe in forcing morality on others.

a: Huh?

b: By seeking to force  your morality on others that “no one should force their morality on anyone else,” you contradict yourself. Perhaps you should be a little more honest and say you think no one should force ‘a kind of morality you don’t like’ on to others. That would be consistent.

a: Ah, um, never mind.