Have you ever seen the Nikah contract? It is a contract and every contract by definition has a termination clause and certain conditions attached to it.
I have a questions for you:
Do you even know what Mut’a is, its conditions and rights and obligations of parties involved? (Not from nasibi sites but from Shi’a sources.)
Once you answer my question, then I will be to explain accordingly.
I hope you will have the decency to either respond or concede with grace and not run away like others have on discussion of Imamat and Ahlul Bayt.