I guess that depends on what you said. If you kept going on and on about how gorgeous Billie Piper was and compared your wife unfavorably, then that would obviously be an unwise thing to do. However, just commenting that an actor or actress is attractive and happens to resemble an ex isn't the worst thing. If your ex wasn't attractive to you, you wouldn't have gotten involved with her in the first place. (Just a guess...)
IMO the prudent thing to do is to reiterate why you're married to your wife and not your ex and why you are still together all these years.
Most of the women I know don't get all weird about that. I've seen guys get weird about that, though.