Sorry OP. So many of them seem to do this now (be in a relationship with one but keep in touch with their exes as back-up's).
I had an ex do this. I broke up with him and he kept in touch every 6 months or so. I was quite open that I was in a relationship so my reply always made that clear and just some general chit chat.
Anyway 2-3 years later I found myself single and contacted said ex. We met up and I was kind of shocked to realise he had been in a relationship with someone for 2 years whilst he was contacting me. Not once did he mention her.
Our relationship was quite short 6 months or so and we were never really good friends or anything. In fact we used to struggle to make conversation when we were supposed to be dating. So it wasn't a case of we were long time friends and so he wanted to keep in touch.
I just found the whole thing a bit grim. I mean why would he contact me and keep his relationship secret?
Anyway I saw him once or twice and then that was it (told him I didn't want to meet again). Over the next few years he contacted me twice and I ignored him. Didn't think much about it to be honest. Then I saw his flat up for sale and he was LIVING with a woman.
Oh and he was a policeman so clearly not the trustworthy sort I had assumed him to be either.
Anyway I could never go out with him again knowing this info. Perhaps it was the same woman and he got back with her or perhaps it was someone new. Either way with them both he was keeping in touch with me. God knows how many others he kept in touch with.
I'm sure he would say 'Oh well I never did anything I was just being friendly' and that is true he never said anything inappropriate. So he either wanted an ego boost, thought the grass was greener, was keeping me as a back-up or was hoping to get me back. Either way I felt sorry for his ex. When we were together I always felt he had one foot in and one foot out and so no doubt he was doing it to me as well.
Another different man who I was dating said to me a few weeks in he was meeting his ex for lunch. I said ok, I trust you. I thought it was a bit weird as they didn't go out together that long and he spoke badly of her. Yip you guessed it. 3 months later his neighbour told me she was spending nights at his flat when I wasn't there. So that relationship was doomed too.
I guess it's a judgement call but if he is the type to talk to exes (especially if he has kept it a secret) then he will probably do it again or with other exes. Hard to say why he does it or if he would cheat given the chance. Doesn't exactly make you feel good about yourself though does it. What did the ex do that the police had to get involved?
Sorry this is happening to you. It's really hard to get trust back once it's gone.