If this is what that guy was doing, it seemed likely he'd become a bigger problem later. At the time, I did not feel a need to confront him. If he became a bigger problem, that might change. What I've tried in the past, ignoring it and hoping the guy will give up, doesn't work very well. So what other options are there? Being an even bigger ass hole seems like a worse option. Walking away from the situation is usually the best option, but it usually not the option I want to take. Dating women that don't attract these kinds of men seems like it should be an option, but I won't know that for sure until I actually date someone that doesn't attract these kinds of men.
The problem is that this women either wasn't really that much into you and/or not really motivated enough to leave this other guy. In this light, any relationship with her was doomed to failure through no fault of your own.
Refraining from acting like an ass was the right choice and it was not the reason that this relationship ended. In other words, you did the right thing and it just wasn't going to work. She wasn't over this other guy and you really had no chance.
The last thing you should take from this is that you need to act aggressively in these situations. If there was anything you could've done differently it would've been better communciation with the girl in question. While you were talking to her in person, rather than just saying "that was odd" you could've asked her more about the other guy, and especially about whether she was still in a relationship with him.
This may well not have changed the way things went but at least you would've had forewarning about what was going to happen!
Ok, purely hypothetical: Lets say things start getting romantic between this woman and I, and the ex decides to start being a dick. Or it's a different woman and a different ex, and whatever woman it is decides to tolerate this behavior. Do I have other options aside from ignoring it or walking away? Saying something like "if this guy's behavior continues, I'm going to have to stop seeing you" seems like a viable option, but I hate issuing ultimatums.
I agree with Robert Maxell
, you should go into a new relationship with fresh eyes. Don't expect the worst!
If there is such a guy in the picture, watch her reaction to him. If she doesn't distance him, well that's a bad sign!
And, like I said above, the last thing you should take from the previous situation is that acting like an ass yourself would improve the outcome! Have an honest discussion with her to see what her intentions are.
Will she distance herself from the other guy? If so, you can work with her to help. If not, well, you're in trouble and really acting like an ass will only worsen the situation. At that point, you probably should be looking for a graceful exit!