He may just say these things to see your reaction - and to test whether or not you are insecure or jealous. If this is the case, he most likely does not care about you or your feelings. If he did he would never intentionally try to make you feel uneasy about how you feel about yourself or how much he cares about you - and only you.
There's also the possibility that he means absolutely nothing by it and he is merely just talking - with no intentions of hurting or upsetting you. It depends on what he is saying about these girls, casual things or are they of a sexual nature?
Regardless of what his intentions are it's best to play it cool and classy. Ignore the comments or change the topic if it bothers you a lot -- but do not get angry or act hurt. You will just be fueling and encouraging him if he purposely says these things to make you upset.
Also, it wouldn't hurt to give him a taste of his own medicine. Talk about another guy the same way that he talks about other women. He will catch the drift in no time and you won't need to tell him that his comments about other women bothers you at all.
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Guys talk about their ex-wife to use them as an example reflecting on something bad that had happened during the course of their relationship and that same something they expect not to happen with their date/gf. It's being too informative (often considered as unnecessary information) when there are other ways of expressing what you expect out of someone but since some guys fail at figuring what that way is, they'll use the ex-wife tactic. This is a poor approach to any conversation, however, only b/c for most women it'll easily drive them away thinking that he still has feelings for his ex and that dating would be too complicated. So guys, if you're the type who indulges on talking about your ex girlfriend or wife, refrain from doing so unless they inquire upon it.