What should you do if your significant other talks about his/her ex?

I would break up with him. Seriously, I would. There is no reason why your husband/wife/girlfriend/boyfriend should be talking about an ex (unless there are children involved, and that is another issue.) If your partner tends to casually mention his former flame's name, it's a red flag.

Sure, you could sit down with him and have a chat, but he'll just deny, deny, deny. You  might hear, "What are you talking about? I don't care about her. I love you." And then you won't hear that name come up for a while. But eventually, she will slowly creep back into conversation.

Say he runs into his ex somewhere. Your man might mention that he happened to see "Sally" earlier that day. End of conversation! If he continues to talk about her, again, she is in his head, and he still has feelings for her.

Remember, if another woman's name comes up (unless it's his sister!) he is thinking about her...period. And you need to get out. You should never accept being number two in any relationship.