H fantasized about sticking his dick in his f colleague, later asks her out for drinks alone.

[FritzDonovan]

Insists he saw her as just a friend. Do men do that, or is it testing the water/wanting a secret thrill/ something along those lines?
She was also acting interested as in gossipy messages etc. Nothing overt. They spent lots of social time in alcoholic situations in a group situation. Then she started messaging him in the evenings too. He doesn't message his other work friends in the evening, and hasn't fantasized about them either. What's his motivation? Opinions from male partners are very welcome too!

Reading your description of your relationship as 'limping through' is so sad. You deserve happiness OP not a lifetime of being emotionally wounded by such a selfish disrespectful bastard. I'd get your ducks in a row and get out.

You sound absolutely worn down, my lovely. He sounds like a shit, he's not worth the grief.
You deserve better, you really do.

He sounds like a grade A selfish arsehole.

Has an affair
Tells you he’s fantasising over another woman
Engineers situations to be with her
Goes on dates with her

Can you not see how completely disrespectful to you he’s being.

He is playing you for a fool and having an affair right under your nose. Get rid.

He doesn't seem to have any respect for you.

It comes across as he knows or thinks he can say/do what he likes and you'll still be there.

In spite of his previous affair when he wanted to leave you ... where he should have been begging forgiveness...he changed his mind and you just accepted it.

That gave him the impudence to do and say what he has in relation to this woman. He's not a safe partner.