It's possible that this wouldn't have happened if you were totally happy in your marriage, but a mistake that a lot of people make is to assume that this never happens even when someone is basically happy and content with their lot. Unfortunately, you see, it does - and much more often than people think, until it happens to them or someone they know. More often than not, affairs happen not because of something "lacking" in a marriage - they happen simply because the partner beckoning is new and the feelings they are evoking about yourself represent a new, or long- forgotten experience.
Hence, what is a simple crush gets blown out of proportion, history gets re-written and the feelings for the object of the crush exaggerated, to epic proportions. People in mid-life especially are often extraordinarily romantic and incautious, all for a crush that will pass if there is some perspective - not to mention integrity and honesty.
The rush you are experiencing at the moment is not I suspect to do with any real feelings for the man himself. It is because of how he has made you feel about you.
It often helps in these situations to try to stand back and empathise with the other people in the two triangles. How would you feel if your H was telling this man's wife, what her husband is telling you? How would you feel if you were this man's wife? Might she say like you, that her H has been emotionally unavailable and under-invested in their relationship? I expect he is right now, just as I bet you have detached somewhat from your H since this man declared his hand.
It is understandable that this man has made you feel sexy, desirable and valuable after all this time. That's pretty intoxicating stuff and very addictive. But you need to see this situation with absolute clarity, because if you both act on these feelings, you could be unleashing a world of pain for lots of unsuspecting people, not to mention the many children involved and ultimately, yourselves.
To be tempted in this situation is human and understandable, just as it would be for your H in a similar situation. But try to step back and see just what you are risking. See too, what you can learn from this experience about you. If it has taught you that you need to feel differently about yourself, then consider happier and more ethical ways of getting that need met. Try to revitalise your marriage. Talk with more honesty than you ever have before about what you need from your H and try to gain esteem from other areas of your life too.
But don't automatically assume that your feelings for this OM must be strong, or that this would never have happened if you were truly happy and loved your husband, because this might be more about you, than your marriage and almost certainly - the other man.