If the guy had followed Letterman's career for any length of time, he'd have realized that Dave's "midwestern Lutheran guilt" complex aside, it goes against his grain to give in to people who are trying to strongarm him or his loved ones. I would have predicted ahead of time that even it embarrassed the crap out of him and wound up costing him viewers, it was not in him to give in to blackmail demands.
He's made no secret of the fact that he never wanted to marry again, half joking that he was such a dismal failure at it the first time. Harry was a huge surprise, but obviously a delight. My sneaking suspicion as to why he finally married the mother is that little Harry got to be school age and started asking when the big event would happen and THAT was Dave's achilles' heel.

But that's JMHO.
Now, if he somehow pressured an employee, that's bad stuff. But unless it happened in the past 6 months, he wasn't married. I have no idea what kind of understanding he and Regina Lasko had, but personally, I wouldn't expect a heck of a lot from a man who made it clear for a few decades that there was no way he was going to marry me. He's long had commitment issues and he made no bones about it. If you expect faithfulness from a man who tells you flat out that he cannot and will not commit, then you're not being realistic.
For all we know, Regina was aware (to some degree) of what had happened and "accepted" it to some extent.....at least enough to stay in the realtionship. It happens all the time. I wouldn't put up with it, but that's me. Maybe he never claimed, "I'd never be with another woman," and she had no expectation of such. We have no idea.
Then again, if I'd been one of his employees, I wouldn't have slept with him, knowing he had a very longtime girlfriend. The odds that I'd take her place would be slim and none. Pretty poor odds.
The best thing about this is that maybe between this case and the John Travolta case, a few more blackmailers will hesitate to twist the arms of celebrities. Sometimes they twist back and the blackmailers end up in prison.