Your first post, "would message very frequently"
You have slightly back tracked now, meetings once a month, completly innocent.
Do you find this man attractive, not just physically but admirably, you met at work, is he still useful to you in some sense ? Why are you allowing him to triangulate his wife, why are you enabling him to have you as his confidant.
This can happen with all ages of men and women, it needn't be about sex although one party usually has adoration for the other. The thing is Nursy, whist you profess your innocense of not being in it for sex you are actively harming and making his wife's life hard. Fact.
We are in no doubt as to where your loyalties lie and I can only imagine how hard it has been for his wife to navigate her marriage whilst his most ardent fan stands on the sidelines supporting his every thought.
How can this man ever be a good husband when he has you to confide in, every argument, every disagreement, every deicision, he has you giving him support and validation, his wife must feel like she can't even be herself, because he will of course compare the reasonable you, hanging off every word he says, to her. Of course you are the cool girl, you have no reason to disagree with him, you don't live with him, have children with him or are trying to be a team making it work in a harsh world.
You are the fun friend who thinks he's great.
You are a carbuncle on their time together, you are a carbuncle on your time together, you are absolutely of no use to this woman's life of trying to bring up a family, I wouldn't stand for it, not for another day, the old statement of mind your own buisness springs to mind, you are his own personal flying monkey waiting in the wings giving him unconditional support.
You are no friend to his wife, you are her enemy, worse than the support of an overbearing MIL who can see no wrong in her spoilt man child son.
The fact he has continued to see you agaisnt his wife's wishes leads me to believe he is abusive and you are enabling that, let him go and teach him to navigate marriage without a 'gang' behind him that bullies his poor wife.
This is not about friendship.