Someone said :
[...] will be read and not ignored.
You realize that there is no way to know if a comment is read and/or ignored.
part of course for the ones you reply to, which must have been read and were not ignored.
Likewise there is no way to know if your father slept with your secondary school best friend, or your mother with your Alma Matter sorority sisters, whilst either might have been dating one another.
As to your original question:
Has anyone else ever been in a long term relationship with someone married?
The answer is "no": they were not in a relationship worth calling it a relationship.
No more in a relationship than a fish wife is with fish knives.
Being the other woman, or the bit on the side, is not a relationship.
Your fantasy is not morally wrong (that would be too easily dismissed), it is ontologically vacant.
As long as you are comfortable with the ghost of the idea of the shadow of a relationship, then do as you please. You are not the one hurting his wife: he is.
And he is hurting you, and you yourself.
If that's your thing, don't let anyone tell you it's wrong or it's immoral or it's not done, or it's unbecoming. It may be empty, but if it is emptiness you seek, you are on the right path.