I tend to agree that if she wants to talk about an ex it's not a good sign that she's having a sexually charged interaction with you. Luckily it rarely happens and most girls seem to at least have the good sense not to do it. If you are going to change the subject, the key is to not make it look like you're threatened or uncomfortable. Think of it as her bringing her ex into the room to try to get a rise out of you. If this happens to me, I do one of two things:
1. Look bored and say, "I really don't give a shit about your ex-boyfriend." (Not in a threatened way just in a 'you're really boring me right now' way)
2. If she's being critical of him, I say something dismissive like, "Doesn't sound like anybody I'd want to hang out with," or I say something critical of her judgment: "You chose to date somebody like that?" and act as though I'm reevaluating her as a prospect. The girl will probably get slightly embarassed, change the subject and start trying to qualify herself to you.
1. Look bored and say, "I really don't give a shit about your ex-boyfriend." (Not in a threatened way just in a 'you're really boring me right now' way)
2. If she's being critical of him, I say something dismissive like, "Doesn't sound like anybody I'd want to hang out with," or I say something critical of her judgment: "You chose to date somebody like that?" and act as though I'm reevaluating her as a prospect. The girl will probably get slightly embarassed, change the subject and start trying to qualify herself to you.