You have already gotten some excellent replies, and I urge you to read and reread them.
"I don't know what to think. Neither him nor I would be in huge favour of exiting our marriages because of the children. I feel sadend that we missed out on a life together, the fact that we have both carried this torch for 25 years makes me think that we must be soulmates."
It is true, you may be soulmates. Some soulmates are lovers, some are good friends. But soulmates do not always spend their lives together and live happily every after. Soulmates sometimes come into your life to help you confront some learning situation, even playing the hurtful one sometimes. What you are saying "Neither himnor I would be in favour of exiting our marriages because of the children." What he is saying is that he has thought of you, but not willing to risk his marriage for you. Of course if you want to have a phone fantasy, he would be willing to play along.
Let me speak frankly. You have been using this fantasy as an excuse not to work on your marriage. Marriages without attention paid to them end up as "loveless" marriages. You owe it to your children to give them a good home with two parents who love them, love each other, and show that they love each other. If you don't you aare teaching them what marriage looks like, no passion, no excitement. Do you want that for your children? So now you have years not paying attention to your sex life, to your marriage, with little sex in your marriage and you have not taken responsibility for it. You have not taken responsibility for making your own life exciting, for making your marriage exciting, for showing your children how to love life, how to make the best of whatever situation you find yourself in.
Your nest beginning to be empty has probably made you think, as we all do at that age. Have I done what I want with my life? Is it the way I want it to be? If you had stayed in Africa and gotten married to this man, you probably would be divorced by now, and you know it. I would bet you are/have been a spitfire, seeing yourself as fiercely independent in your thinking, and unable to be dominated, and there has been a desire to find a man who would match your fire, who could make you feel as you want to feel, fiery, filled with excitement. Your husband's like of fire has left you disappointed, depressed. You turn to the other person in your life who you had fire for, and you wonder if it is still there. You find out he is not in a perfect marriage either (not to say that any marriage IS perfect) and now you are thinking if you still could get some excitement in your life through him.
Alcoholics speak of "doing a geographical." That means they move to think that things will be better somewhere else. Trouble is, they take themselves, as they are, with them. So the next situation is relatively the same, just a change in faces and names.
I urge you to, as Dr. Phil says, "get excited about your life." That involves a change in attitude and taking responsibility for letting your marriage get into the shape it is. It does not involve a geographical. It involves making your life as exciting as you seem to think it needs. I would say it has been easier for you not to have that excitement or you would have done it already, less work for you as it were. The payoff is that you can blame him for a loveless marriage, when in reality, it was you as much as him.
Now if you do not take responsibility for your life and your marriage you may show resentment in some of the inflammatory diseases, arthritis, bursitis, gallbladder problems, any disease that shows crippling, because you feel that this marriage has crippled you, or some other metaphor. You may gain weight so as not to get close to people, living in your fantasy world of what might have been, watching too much tv, getting involved in soap operas, whatever your drug of choice.
In the end, we have to work out our emotional situations or take them into our bodies. If you need excitement, go to a ball game and scream and get excited.Grab your children and hug them, get interested in their lives and help them make their lives exciting in a healthy way. Learn with them; I learned myself while looking for things to enrich my son's life and to have him not grow up as I did--I did not have the tools, so we learned together. Have a talk with your husband about your need to have more excitement, beginning with date nights and with the goal of just rekindling the love and passion, not necessarily watching a p 0 r n flick together. Do what you need to do to enliven your life where you are, without contact with the ex so as not to have that pull on your back into Fantasyland.