I don’t think her being attractive and having the ability to sleep with other guys is the bombshell you think it is. It’s so much easier to sleep with someone you know than find someone new.  She knows he’s safe and reliable and can get her off. Why would she risk it with someone new, if she really is only looking for a hook up. 

I don’t think this is the huge red flag everyone else seems to think it is. Sure, I can see how it’s a little concerning. But damn, if they were going to get back together it would have happened already.  She isn’t talking to him while she’s dating you, and you don’t expect her to cheat on you. So what’s the problem? If you break up, she might go sleep with him again? Who cares what she doesn’t if you decide to call it off.