General Discussion

It was extremely important that the campaign manager know about any skeletons
before running a campaign. They hate getting blindsided.
You have to stay ahead of the bad news.

And it is naive to think the MAGAs would not have found out about this on their own.
They hire detectives to scrape out the tiniest bit of dirt on Dems.

The answer to your other question:
The sex drive in males is incredibly strong (see Darwin for more info on this topic).
And with some men of weak character/ limited intellect/ poor judgment,
they act it out even when it means the destruction of everything important to them:
marriage, family, income, job, country, etc.