I think in this situation you have to put your foot down. If I'm in a relationship I refuse to have ex's in our lives. You are still in the early stages, so she will be testing you in all kinds of ways. They tend to do this to qualify you and if you are going to let an ex steal her back, then you are simply not man enough and fail the test... as harsh and unfortunate as it may be. You need t make it clear what you will put up with and what you wont... or she will walk all over you.

Display the fact that you will walk away if she crosses the line, she will either submit or fight you on it (Most cases fight, using emotional ploys and what not)...but you have to be willing to do or she will see you are full of shit and take advantage of it

If getting laid is a concern, go through all this stuff while getting girls interested on the side, that way you have something lined up and don't get dropped into a dry spell, which may cause you to cave, give her all the power. This is also useful for showing that you are capable of leaving and finding someone new on a moments notice... And if you get caught up, just say they are friends... Chicks do it all the time.

Basically Step up and put your foot down. Be willing to take what is yours and walk away if need be... this dude is not playing by the rules and swooping on your chick. There is a valuable lesson to be learned.