This is a really sad story and I feel for you. I'm sorry you are going through this. You are fortunate that she is talking about her sexuality.
"She says as a middle aged mum she wouldn't even know where to find someone else. She says she would like to but that it seems an impossible and intangible thing to achieve."
I don't believe this for a second. That's a suspicious statement to me. 

I'd say you have already suffered enough rejection and hurt throughout your marriage, to stay together would be setting yourself up for more pain. Especially because she wants to have sexual relations elsewhere with women and has said that sex with men disgusts her. How could you honestly handle a relationship without slowly dying inside? How lonely and dismal for you.
I'm a few months ahead of you from d-day and I was just as empathetic in the beginning. I think it's shock, our 'unconditional' love, you get into survival mode because your family has been blown up. Now I'm raging because reality is setting in. Having to rewrite my future because of this person that was not honest from the beginning.
I'm sure they know they're gay from a young age, I just think that fear stops them from living that life.
I also think fear is stopping your wife from leaving. Of course it's comfortable to be in a family unit, everyone wants that coziness and taking care of and relying on each other. Especially financially, let's be honest. But what about when she falls in love with a woman and decides to end the marriage? You will be even more devastated than now. 
We need to be extra careful where we invest our love and affection. Especially now that we know the truth. In these cases it feels like all the efforts, and carrying the marriage, were a complete waste of time and not appreciated.
How do they sleep at night knowing how much they hurt us?
I hope all goes smoothly for you. Please be prepared for a very 'rocky road' ahead. Emotions run high and you both need to find good therapists.
My psychologist and I are working on my voice, finding it after I've been silenced all these years. Don't be afraid of her reaction when you tell her how you really feel, I mean really feel! You need to get out the rage (you don't sound angry yet but this is coming).
Take good care and keep us posted. Your daughters deserve the truth, everyone does. I don't believe in secrets and I know for a fact kids do better when they have the truth rather than wondering WTH has happened to Mom and Dad. I truly hope your wife is willing to be honest with the girls and everyone around you. Please don't cover up for her and get locked in her closet.
 

My apologies for my post in this section. I thought it was in another category and not 'Strategies for MOM's' 
Should I delete?

Hi Rocky. I read your post and it is almost a mirror of my situation. My Dday was July 12 of this year. you can read my story in the "My Story section " . We are still married and have a 9 year old son. My advice is to take it one day at a time . I was devastated ,I had no idea she was gay, married 22 years together for 29 , and she tells me that she never loved me the way that I deserved to be loved. We are best friends and soulmates. But as you get some time under your belt , your feelings and thoughts catch up and the reality of your situation really sets in. The range of emotions is almost unbearable. Can your marriage survive? I don't know my brother....I am in the same shitty boat. I could have seen myself being celibate before my Dday but for different reasons. Her being gay has kind of changed my thoughts on that. Give yourself some time to process this. My wife has never been with a woman and is not actively searching( so she says). In the end,,,,does it matter?  All the best brother, you are not alone.   Steve

Rocky road,

it sounds like you're in about the place I was in 15 years ago. I recognize a lot of similarities between our stories.
I suppose you know already about the (rocky) road I/we traveled. 
We eventually arrived in a very good and fulfilling (monogamous) relation, also sexually. Very different and better than it ever was before, this is no exaggeration. So there potentially does exists another route then "open relations", celibacy or divorce.
My story is mainly written from my perspective, in further posts I tried to clarify and also supply info about somehow my wife went through it. Later my wife (Samantha NL) also wrote some about this on this section of the forum (15 years in my MOM with Dutchman).
In what we write about our experience you'll undoubtedly understand it's not an easy trick. Main thing: it requires love AND choices.
This is definitely not (!) concerning choice about sexual orientation, but rather the choice for the relation and for each other. 
If you both choose to go for a MOM, in my opinion it should be (ultimately) all or nothing. 
Either divorce and call it quits, or go for the goal of a real and complete marriage relation. This also includes a relation that is sexual fulfilling to both, so no in between solution where one or both are gradually broken down.
But you can't jump from zero to a hundred. It all takes time, during which you'll most probably seek and suffer both, and go through highs and lows.
First couple of years are worst, and the quest will take years to complete. Look before you leap, it won't be easy.
Having said that, it can be worth every teardrop you've shed. But that's afterwards and looking back. We both went for it, surely stumbling and making mistakes along the way, going through a misty fog, but we both wanted it.

At the moment your world is turned upside down. You wish to find a way through this. But it's not just your decision. You both have to make a choice.
I get the impression your choice would be to go for it, try to make it work, and meanwhile truly accept your wife as the person she is. But your wife has to make her decision. Not only for herself but also includes you, your health and happiness.
Acceptance must come from both sides. Just accepting herself (and coming-out) is just half of the story when in a MOM. If she wants to continue the marriage (choice), that means she has to accept you as well. 
You're her husband, who suffers the non-existent sexuality in your marriage. Keeping it that way equals not accepting you as you are. So... why would she expect she should be accepted and supported as she is?
She'll have to address this dilemma for herself. For some years already she didn't (want to) realize this fact. But now she came-out and I suppose wants to be honest, she can't avoid this question anymore.

It's not about expecting to have great sex next month, but it's about her mindset! Is she willing to care and think about your wellbeing (as a man!). 
Is your importance on her radar? Because exactly this attitude tells a lot, and predicts a lot what's down the road.

Hopefully it's clear what I mean. Because actually restarting the sexual relation is a delicate matter, and also try to get professional help for this issue.
But the main thing now is her stance in relation to you (and something she really has to consider carefully) : is her will directed to work towards you, or is her sexual orientation actually the paramount thing that's really important to her?
If the latter is the case, you'll better let her go with "this thing" she desires most. 
But if you are the goal of her heart, then both can make the considered choice to go for it and strive to make it work (and it can).

RockyRoad,

I am very sorry you are going through this.  It is very traumatic!  Sex is an important part of intimacy.  It sounds like you are very good friends and you love each other.  I am in a similar situation with my bi husband, only he did have an affair with a man.  He wants to stay together and says he will be monogamous, but I fear sooner or later he will have to explore that need.  I also want to have more intimacy and sex and he is hot and cold.  I think you deserve to have a strong sexual life.  I realize now that the problem with our sex life is my husband.  He can blame me, but it really is his problem.  We are trying to see if we can come to some sort of arrangement too, but just don't know.

Give yourself time.  You will cycle through many emotions, some are very dark; however, if you love each other, you will work it out together and time will reveal what you really want.  You are probably in shock right now and I am very sorry for this.  I wish you much healing and strength.  Bless you.