Complex sentence: When Stacey finds out Martin is planning to propose, she says she does not want to marry again so he proposes that she become his "not-wife", so they are committed and she agrees.
Split into simpler sentences: When Stacey finds out Martin is planning to propose, she says she does not want to marry again. He later proposes that she become his "not-wife", so they are committed but not married, and she agrees.