I have the utmost sympathy for your position. I've been there myself. My ex had a relationship before our marriage ended, and I had absolutely no idea who he was dating. I knew a tad about her (they went on a business trip together when they hooked up and I talked to her a couple of times before then, having no idea about her lack of morality
), and generally have gotton little information about her. And she's got two nearly-teen-age boys, which scares me to death, but I can't do ANYTHING about it. And I've found that in life, when you can't do something to make your worries go away, it's just not worth worrying about.
Also...from the other side, when I started dating someone seriously, my ex had an inkling because of some things my dds had said. Yes, my s/o met the girls before ex even knew about him...in part because we met first as friends, and in part because ex lives so far away (1400 miles). But ex waited until I was ready to tell him about my new relationship...and that made a huge difference. No, I don't trust him at all. He's been a horrid jerk to me. Why should I want to share info with him AT ALL? But I do respect him, and I struggled with this (there's a thread in single parenting about it, should I tell ex about a new relationship?) In the end, I felt comfortable sharing some info with him, and some things I just said, "sorry, that's none of your business." or "you're entitled to your opinion, but it's my life and my parenting choices."
So I'm just trying to give you some perspectives. I've had to rely on the serenity prayer in my divorce...Lord grant me the serenity to accept the things I cannot change, the courage to change the things I can, and the wisdom to know the difference. I hope some of this helps you have peace.