It's funny, last night my partner was asking about a sexual relationship from my past from before my gender transition (when I was living as a man, rather than as a womon). I hadn't talked or even thought much about it in a while, but it was probably my most significant experience in terms of exploring an ongoing relationship with more than a couplehood.
So like you, I was the "husband" at the time, and then there was the womon I was married to at the time, and the womon we were dating. My partner and I were discussing last night what had gone wrong and how the relationship had ultimately fallen apart. Part of it was circumstance, but there was also jealousy. My conclusion (some ten years now after the fact) is that I played a significant, yet unrecognized role in destroying everything by my attempts to "manage the situation."
Raised as a middle class, white "male" in the culture of American suburbia, I was so entrenched in a culture that was conditioning me from birth to "manage" and "administer" the world around me that it was a condition I was completely oblivious to. IOW, what is the ocean to a fish who has never been out of it? Having recognized the condition, I can't even say that I try to avoid it, since that would almost be a kind of management in and of itself. I just recognize management for how I see it: authoritarian.
I won't have to live with the decisions you make, so I wouldn't presume to tell you what to do. I'm just going to say that I trust you can figure it out, if you let go of your ideas of "should" and focus more on observing what "is." I don't try to work out long term strategies any more, I don't scheme, I don't try to say everything perfectly, I let myself fuck up, I let myself apologize for fucking up, I let people see me as human, I let people see me as they need to -- in short, I don't seek to interject myself into the situation. I let things "work themselves out," although I also recognize I am one of those things working itself out.
Life is complicated and sex is a very complicated aspect of life. The more I try to "figure it all out" consciously, as if there are 5 steps or 12 steps or 11,342 steps that can be written down and committed to memory, the further I get from "truth." Learn from these wimmin and develop your *respect* for them (and their intuition), love will follow the respect, and then feelings of love will lead to acts of love.
I'm sorry to sound all Deepak Chopra, but if you just get your idea of how things "should" work out of the way, you may find that what needs to happen will come much easier.
As an example, if the opportunity to present itself comes up in conversation (it finds you, not the other way around), you could try just saying something to your significant other such as, "sweetheart, you know I would never be unfaithful to you, but you know _____ that we work with, and I really think she's awesome. I wanted to say something because I hope that's ok, and I wouldn't want the two of us to be too friendly and then you either had to wonder or else have your feelings hurt."