A lot of my fears are based on her finding someone else. Really tough for me to envision that.....or envision another man spending time with my son in a "quasi-parent" role. It's tough to imagine her with someone else. Nothing I can do about it, though.
Here's the thing about fears, if you go back and look at your life you'll probably find that all the bad things that happened to you were probably things you weren't expecting, they happened while you were afraid of other things. That was certainly the case for me. I spent so much time worrying about money and my job that when I was blind-sided by BD it was a double-shock, because that was never even on my radar of things-to-worry-about. In fact, I always said to myself "well if all this other crap happens, at least I still have my M!"
My W has an OM, I have no idea if they're romantically involved but I do know that they do all kinds of stuff together both alone and with my kids. I actually know him a bit from pre-BD and he's a nice guy. Has a lot in common with me actually. He has always been very kind towards my kids and despite my sitch I'd have to say if my kids have to be around an OM then he's not a bad choice. Unless your W has gotten wreckless then you can probably trust that if she gets in another R it's going to be someone similar to you, and thus someone that might be a good influence. As long as you keep up your R with your S the OM will never replace you (which is what I think many of us fear most).
