I don't have any great insight for you, but I have had a somewhat similar conversation.
My best friend is married, straight and mono. She has always known that I am bi and poly. BUT when I got involved with Dude I didn't tell her right away (my prior poly ventures had all been with women - and never evolved into anything more than FWBs - this was different).
It turns out that she knew something was up (I'm a talker, and Dude kept coming up in conversations ... as a "Friend"). When I was ready to have "the conversation" (6 months in, or so) it actually went fairly well. She had some questions and concerns and I answered them to the best of my ability.
I don't think that she was really, really "ok" with it, though, until we (the three of us) stopped to stay with her last year. At that point, she was able to SEE that MrS was perfectly comfortable with the dynamic and happy - and her fears seemed to visibly evaporate. (She really likes and respects MrS - most of her fears were that he was "putting up" with something that was eating away at him inside because she knows how much he loves me. Her main concern was that I would hurt him and screw up our marriage.)
Her husband...NOT okay with it (although he was perfectly pleasant and hospitable while we were there - his negative reaction will only ever be expressed to her, never anyone else). Nor would I have expected him to be, nor do I particularly care. Just means that when she comes to visit US, then he won't be coming along.
Here's my dilemma. I haven't told her about my 4 month relationship with my gf MD. I have told her that my husband, FJ, and I have discussed the possibility of me fooling around with a girl on the side. She knows I am bi. We have very candid and frank convos about all types of sex and our own sex lives. But when I told her the FJ didn't mind the idea of me and another girl she couldn't wrap her head around it. Doesn't understand how he could be ok with me "dividing" my love and attention. I have felt judged by her at times, although it usually is nothing-just her protectiveness over me.
I have to tell her about this part of my life. I know she'll be hurt I haven't told her for so long, and I'm afraid of the fall out. Any suggestions of what to say? She is a very thorough questioner, so ill be bombarded with a million questions.
Ack!
I would preface the conversation by referring to some of this - along the lines of:
"I've been wanting to talk to you about something important that has been going on in my life, but I was afraid of how you would react. We have talked about my bisexuality and that FJ and I have discussed me being with another girl. You may have noticed that 'something was up' recently - it's that we have forged ahead on this front - I have been seeing MD for the last 4 months.
I know that you have concerns - you have expressed some before, but before we get into it I want you to know that I love you and value your insight but that FJ and I are just fine. This is a new experience and there are some wrinkles to iron out (time management, etc) but we are learning and growing. MD and I are so new, we are still figuring stuff out. I have really wanted to be able to talk to you about this but I was worried that you would judge us - even though I know that it is just because you care about me."
At this point - if she doesn't yell that you are a horrible person who is going to burn in hell forever and that she never wants to talk to you again as she slams down the phone - she will probably have a LOT of questions. You won't have all the answers - that's ok.
If her questions have to do with how FJ "really" feels - you could say: "We have talked about it and he tells me x-and-y, his actions back that up, so I believe him." and offer that she could talk to him herself.
If her questions have to do with how this could affect your children - you could say: "We have talked about this...Obviously making sure that our kids are not harmed is a very high priority. So we have made x plans regarding that (making sure that they get quality time with you, etc.)
I would expect a lot of "What if" type questions - you answer those depending on what they are:
"Well, that would be a possibility even if we were just friends - I don't know that dating someone makes that more likely."
"Well, we thought about that, and if that were to happen then I think we would respond by doing X"
"Well, that is an interesting question...I actually hadn't thought about that before, THIS is why I like being able to talk to you about this...my gut response is Y but I think I need to talk to FJ about this some more."
If she stops asking questions and starts accusing/judging you? (i.e "I can't believe how selfish you are being. Your husband is only saying these things so you don't leave, how can you DO this to him. You are a bad mother to do this to your kids." etc.)
My advice would be to dis-engage and walk away for the time being. "I'm sorry that you are responding this way - I was afraid this might happen, which is why I didn't come to you sooner. I want you to know that FJ and I are comfortable with our decisions, we love each other, and this does not change that. I'm going to leave it at that for now, and if, in the future you want to talk about this again - once you have had time to think about it - then I am open to that. But we all have to decide for ourselves what is best for us....Goodbye."
She might need time...or she might never be okay with it. But I wouldn't try to argue and justify things if she responds negatively to your initial disclosure. You will both end up saying things that could hurt the other, that could actually permanently alter your friendship - and that is so NOT the point.
JaneQ
PS. If push comes to shove you can point out "you don't HAVE to understand, you can just ACCEPT what I am telling you"