Jane Hamsher’s right:

It’s about his relationship with a telecom lobbyist who boasted about her ability to professionally influence him in business situations, and whose lobbying interests McCain has taken clear action to help.

Or, to put it differently: It’s not about the sex, it’s about the corruption.

…Though I do think that if there is documented evidence or first-hand, on the record accounts of how this inappropriate relationship with a woman who is not his wife, it would be valuable in removing the false patina around John McCain as an honorable man of upstanding principle. He’s not and beating back that narrative is important. A bona fide sex scandal wouldn’t be valuable for the sex, but for the impact a scandal of that nature would have on his image.

Update:

Or, as clammyc puts it, “It’s the credibility, stupid”