Re: lingering questions in my mind
you can't rationalize this. What she did is not logical.
Having sex with people usually does lead to bonding. It is why having a sex life helps married couples - it bonds them, through chemicals released in the brain during.
If she is still in contact with this dude, it will most likely happen again... and again.
She should be trying to move mountains to get you to stay. You need to impose some consequences on her - inform the OM wife, her parents, your parents, a trusted mutual friend or two. She needs to ditch the current job.