So her excuse for being okay with Tav cheating on her is entirely dependent on encounters she never enjoyed or wanted to begin with? You're not really doing anything for your case here.
If she didn't like or want the casual sex and was prohibited from having an actual relationship, it only makes more sense she would choose monogamy with Tav.
Truth be told I'm not making a 'case'. My position is indisputable. SH dates more than one person at a time - you can like that or not but it just IS. I'm just getting at the why of it.
Tav cheated, she let her know that it would have been okay if she had just asked. Which is a pretty generous response and, yes, this is a *very* poly conversation. Cheating can happen in a poly relationship - poly is not free pass.