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But she assumed—incorrectly, as it turned out—you would have a problem with those texts.
It was a reasonable assumption on her part, since swapping flirty texts with a stranger is regarded as "out of bounds" by most.
So, yeah, stop answering that drunk girl's summonses.
While I would be okay if she were doing this and I knew about it, this has been going on since before we met.
But if you would have been fine with this had you known—if there was no reason for Mary to hide this LTR-of-sorts from you—the best way to prove that to her is by giving it your retroactive blessing.
You're right, UITM: Mary shouldn't have hidden this from you.
Prior to this, it never occurred to me that Mary would do anything that had a whiff of dishonesty about it.
But her having kept this from me for as long as I have known her has made me question that.