I don't think you have any obligation to let him know what you're THINKING. I also don't believe you're under a moral obligation to let him know if you're going to date, if HE is having an affair, lying to you about it, and actively moving (as in filing), or if YOU have filed, to end the marriage. Some people decide, morally and ethically, that so long as they are still married, they won't do this, and others only maintain that stance up until a certain point -- when they are "done" -- and they begin to move on. Many reach that point when a divorce is FILED, but not yet COMPLETED. That's something only you can answer for yourself, in your own conscience and belief system.
So you really don't need to say ANYTHING, since your husband still isn't being faithful (I assume), and still isn't being honest with you. However, I would still suggest you be forthright about this, and I'd do it in the form of "goin' Robx" on him, like:
"(Husband), I've been thinking, and I've decided that this really isn't working for me. I don't want to be with someone who doesn't want to be with ME, in a committed and monogamous and transparent way, and you're obviously not ready to commit to that, by your recent actions and indecision. I think we should each get some legal advice, and I've also decided to begin dating other people. Although you haven't really been honest with ME, as a courtesy, I wanted to let you know."
I think it's important however, AMG, that you do NOT do this just to get a "rise" out of him -- this has to be what you really decided. And I'm sure others will post that for THAT VERY REASON, you SHOULDN'T even tell him, and you should just DO it, and I'm not even so sure that I disagree with that.
This subject (dating other people) is one that, like filing for S or D, sadly WORKS, but then you're left with the position of "Great -- so the only way I can get my spouse's attention is if I date other people, and file for divorce????"
