I recommend refraining from flirting at work. Even if he was single it’s better not look for future partners at work.

I’d say if he asked you out or made other passes on you, it would be leading you on but if he just looked at you or laughed when you regularly visited his desk or even got coffee together, I am not sure it’s called leading on.

I wouldn’t try to look into what’s wrong with his marriage. There is likely nothing wrong. They might not be into pda or might be just those kind of people in public. I’d say if he had bad marriage he probably wouldn’t even bring her. But it doesn’t really matter. A man is married and at no point suggested you two have an affair ((bad idea regardless)

Just focus on your work task. I don’t know what you do for a living but it amazes me how much time and energy people have at work to be able to socialize that much. Does this job bore you? Maybe there is something more suitable out there that would get you so engaged in work task you’d not notice or care if men at work looking at you or not