In the story, M+M, how is Martin's failure to tell Mariela about his communist leanings a betrayal of her trust?

In the imagined conversation, Mariela accuses Martin of betraying her, saying “[he] didn’t tell [her]” (60) about his affiliation with the communist rebels. Martin’s association with the communist rebels is especially significant for Mariela, because the communist rebels attack Mariela’s father, whom Martin says is a “crook” (82). Thus, Martin’s failure to tell Mariela of his communist leanings and his eventual joining of the communist army is a significant betrayal of Mariela’s trust.