Jools - your situation is different to Popzie's and so I just wanted to add my thoughts to your quandary.
The first thing is that old flames are just the worst people to have affairs with. Your feelings for the OM are inextricably linked with nostalgia and a yearning for your younger self, before marriage, mortgages and babies got in the way. Please recognise the delusive qualities of this and do challenge any "he was the one that got away" fantasies you might be having. Or that he might be having, for that matter.
Secondly, allow me to view what you are saying analytically. You are basically telling us that you have lived the greater part of your long relationship with a man who very rarely wanted to have sex - and now doesn't want it at all. That you have been honest about how this makes you feel and that he knows this is a big issue for you. That from what you're telling us, he won't seek help for his zero libido.
I think sometimes we get so immersed in our situations that we think there are only alternatives and not the vast array of options that are really there. You believe that one option might be to leave him and form a new relationship. You see the alternative as living a life without passion and intimacy for another 40 years. In fact there are far more options than this!
It's good that you are starting to see that more honesty with your H is needed here. I suspect he has buried his head in the sand before about this issue and is too complacent to think his choice of celibacy might cause you to look elsewhere.
Jools, if you're being honest with yourself (and us) here in that you do want to stay married to this man, the first thing that needs to happen is that your husband wakes up to what is actually happening here - make him take it seriously. Withholding sex and intimacy is unfair and a deal-breaker in its own right. But so is infidelity and deception.
I understand that your desire has gone for him too, but desire can return, especially if it was once there. Given that you have wanted more sex with him in the past, I'm assuming you once did desire him very much.
Your H needs to know that unless he seeks help (medical or therapy based) about this, he is going to lose you. And he needs to believe that this will happen. Once he realises that his choices in life (and they are his choices) have consequences, he might decide to get help. You then decide whether you'll support him in that journey. The honesty between you then can kick-start your marriage revival - it really is incredible how much more intimate you feel towards a partner when there is honest communication.
On the other hand, he might say he won't get help and then you make another decision. One of these might be an open marriage with his consent, or leaving him and pursuing other relationships.
I'd challenge your view that: "I guess that the fact I am even contemplating being with another man means my marriage is over. How can I feel that way about someone else if I really love my husband?"
Of course it is possible to love your husband and feel this way! Please don't do that female thing of assuming that your marriage must be doomed if you are considering sleeping with another man. From what you're telling us, this boils down to a couple of very specific problems - you don't want sex with each other and there is no desire. Address those issues and don't write off all the other qualities of your marriage.
Living a life without intimacy is a rubbish option for most people and it is for you, which is the most salient point. But affairs and especially deception are a rubbish option too - what ever people might say on here, that affairs are a fact of life etc, this conveniently neglects the issue of deception. As Gonna said in her heartfelt post - it is the deception that hurts the most.
There are other options here Jools - but it needs bravery and courage to take them. Don't box yourself into a corner on this by believing that you've got limited choices.
Where your situation crosses over with Popzie though is that despite what Popzie prefers to believe, these situations never "resolve themselves". Not without enormous hurt once an affair is discovered - or the dull hurt of two people living in a passionless marriage.