Story of Bob, Mary and Molly could have one of several issues
Q: If Mary and Bob are in a happy, stable, two-year relationship, would it ever be appropriate for Bob to go out for drinks with Molly, a woman he went on a few dates with in the past (nothing serious), and who knows of Mary's existence, but hasn't actually met Mary? (Molly and Bob aren't really in touch outside of Facebook updates.) What about a "once-in-a-lifetime" event, like courtside seats to an NBA game, for example?
Mary could presumably join them for drinks, but not for a game because Molly only has two tickets. What about then?
Mary thinks Bob might be leading Molly on, and is uncomfortable with the situation. Bob says it means nothing to him, and what difference does it make how Molly feels about Bob if he is in love with Mary? Bob thinks it's a lack-of-trust issue. He thinks if he tells Molly that he'd like her to meet Mary prior to the game, it would be sending a message to Molly that Mary is distrustful of Bob and is "supervising" (not Mary's intention). Mary wants Bob to send a clear message to Molly that she has no chance with him. How to proceed?
A: This is like having food go bad in the fridge — you know right away that something stinks, but identifying it means peering under various lids.
Something stinks here. That's because committed guys say no, drama-free, to invitations from people they don't care about, and they say yes, drama-free, to true platonic friends. Instead, your hackles are up, and the story doesn't track; if Molly "means nothing," then why accept her invitation, much less care what she'll think if you join them for drinks?
I can tell you, though, that Molly isn't what stinks. She could be home lighting candles at her Bob shrine and it won't matter if Bob isn't interested.
But that brings us to one rottenness possibility: Bob is just using Molly to get courtside. If nothing but this situation has ever given you reason to believe Bob wants Molly, and if Bob has user tendencies, then there's your bad smell.
Another possibility: You've had problems with jealousy, both in this relationship and in past ones, and you're primed to see rivals where there are none. It's not unheard of to downplay information that suggests this is a harmless outing — say, Bob and Molly have a real friendship, and a "few dates" long ago were sufficient to rule out a romance — and dwell on "He's going out with an ex."
Another possibility: Bob wants Molly, and he's using cool tickets to justify an exploratory date. If you've never had problems with jealousy in your relationship with Bob (or in general), and if there's something about the way Bob talks about Molly that makes you uncomfortable, then trust it.
People can be users, or controlled by their own jealousy, and they can have a long history of auditioning Mollies while still dating Maries. So, figure out which one you've got by asking yourself: Is Bob's version of events credible?
Take the last two years as a collection of dots, and start connecting. Trust what you know, and trust what it tells you to do.