Co-worker issue...beyond confusing?

Okay so I am 19 and work as a receptionist in a real estate office. There's one realtor that stands out to me. We joke around and flirt at work. There's a couple problems...he's 20 years older than me. I know the answer to my issue must be obvious, but its not. We constantly make puns at each other and the flirtation grows as weeks go by. We almost kissed today at work and the worst part is that he's married with children, 3 to be exact. His marriage is on the brink of divorce and this isn't because of me, its been going on for a while. The only thing that I have an issue with is the fact that I don't know how to handle this situation. The flirting is strictly between us, no one knows about this at the office and I am seriously not sure what he's thinking. He acts like a total dweeb and then has this sweet caring side to him aswell. He helps me a little financially but for little things that I can handle but he pays for me anyway. So anyone just give me an idea to this, what should I do? Im not falling for him, but a part of me is really attracted to him.