Thank you all for your answers.
Writing about such a complex topic is not easy, and you have to focus on certain aspects while neglecting others, so some things may not have come across exactly the way I intended them.
First of all:
For all I know, my husband has been candid with me all the time.
Three days after he had met her in the internet (stumbling across her in a general forum - not searching for somebody in a BDSM community or something like that) he told me about her and asked if he could be her online dom.
And I also knew that they would meet.
I knew that he couldn't give her up. He didn't lie to me about that. He didn't say that he had ended the relationship or something like that.
So, there is honesty. We do talk. Actually, we've done a lot of talking.
And I still trust him.
As for her and the sex. She lives in a open marriage and has sex and/or SM (not D/s) with several other men on a regular basis.
I believe my husband when he says that there is no sex planned. He wants to be special for her, not just another fuckbuddy.
As for the aspect of neglect. I don't want to go into detail there, because that's a very complicated story, too. Just one thing:
I don't think that the lack of neglect alone proves that everything is ok. I guess a man who cheats on his wife with his secretary during his business trips isn't necessarily neglecting her. Perhaps even the opposite. But still most people would understand that she was hurt if he did that.
Yes, I know that it is nobody's fault that he has needs that I cannot satisfy.
But, well, we've lived for almost 20 years in a mutually exclusive relationship. That's what I bargained for. That's what I'm used to. That's what I want. That's what I need. And it is not my fault that I need that, either, is it?
I know: I will either have to find a way to come to terms with the fact that he has somebody else, or I will have to leave him.