Boyfriend talking to an "old friend"

I have been seeing my boyfriend for six months now and it has moved pretty quick considering the time we have been together. We are already living together and pretty serious. he is very open and honest about everything and for the most part we have a very happy relationship. We are still getting to know each other and both have our baggage but who doesn't. He use to be in the military and has a few friends from back then that he talks to every so often. He recently got in contact with a female friend from back then who is now married with kids but who I know he has somewhat sexual relations with and strong feelings for in the past. Even when she was already married he still made attempts so the fact that she is married doesn't comfort me. They have said little things such as I miss you and want to hangout again. He has told her about me but I'm not sure how much. I told him that I was somewhat user of how I felt about their relationship and that I still have no problem or rite to tell him that he cant talk to her. I don't like to be that type of person but since then he has been talking to her and not telling me and making a point to tell me he hasn't been talking to her when I know otherwise which makes me feel like he is being sneaky which I don't like :( Either he is just not telling me to avoid making me upset or avoid conflict and there's nothing more to it or he is hiding it because there may be something more to it. I just don't know....