The bigger story is not sex, but influence peddling. McCain wrote two letters to the FCC on behalf of the firm his alleged paramour represented. For someone who has cloaked himself in being a reformer and against special interests, this is especially damaging.
K street hires pretty blond.
Old man likes flirting with pretty blond and helps her do X.
The K street firm could have just sent him to a strip club but that would look odd for a senator.
If you write a letter to your Congressman complaining about some problem you are a having with U.S. Government Agency X, someone on the Congressman's staff will send out a letter to that agency regarding your problem with the Congressman's letterhead, name and mechanically reproduced signature on it.
If this were a crime, every Congresscritter in Washington, DC would be guilty of it several thousand times over.