I am touched to be addressed as "Professor," but perhaps we can transgress the public-private boundary to the extent of using first names, even though we meet only electronically.
Let me react to your three objections in their strong form, which you later qualify--since they present an important view.
Hypocrisy: Strictly, this means professing values you do not hold and denouncing others for conduct that you engage in yourself. To answer your first question, that kind of hypocrisy deserves exposure, even if it means publicizing the sex lives of moralistic evangelists and politicians. Helen Chenoweth got what she deserved.
But you include under hypocrisy something else, which is also a problem of American politics--the standard presentation of the candidate as human exemplar and glowing family ideal. I agree with you that this has provided an opening for media intrusiveness, and think we would be much better off if politicians never dragged their families into the limelight. But they are now stuck with the practice, whether they like it or not, and the public-private boundary has to be drawn at a different place. If it is generally recognized that the familial frame in which politicians are presented is a convention, there is no point in exposing it, any more than there is a point in exposing the social conventions of politeness as dishonest. Kennedy was packaged as a devoted family man. Do you think the public interest would have been advanced by his sexual exposure?
You suggest a blanket requirement of truthfulness, but anyone who has been around for a while knows that the unvarnished truth at full volume is tolerable only in limited doses and that a great deal is better left unsaid or covered up. Perhaps you wouldn't trust someone who lied about his sex life. I wouldn't trust someone who didn't, some of the time.
Exploitation: This is precisely one of those issues that is forced on us when intimate relations become public, but which there is no reason to settle publicly. My view is that while people often behave badly in sexual relations of any kind, as they do in familial relations, no generalizations about who in such cases is exploiting or maltreating whom are worth anything. But the real point is that our different responses to this kind of story, strongly opposed as they may be, are irrelevant to political judgment, yet they crowd out what is relevant. Visceral feelings of either sympathy or outrage with the sexual couplings of middle-aged statesmen and their breathless groupies are just personal reactions and will take us nowhere. Only a pathological communitarianism would insist that we come together as a society to arrive at a collective conclusion about the right attitude to such an episode. As with truthfulness, you suggest that there is a single vice called "using" people, which generalizes across contexts. But people are more complicated than that, and we already have enough on our plates in trying to evaluate the public conduct of public figures.
Democracy: As you say, the general public seems in the present case more sensible than those people who modestly call themselves the "elites," with respect to the political relevance of sexual information. But you pose a difficult question about the role of the press in a different climate. If puritanism is culturally dominant, should the press still adhere to the convention, one that serves the interest of all parties, that private embarrassments will not be aired for political gain? Of course a puritan climate will help sustain that convention by directly inhibiting the press.
I don't have a theory of the ethics of journalism, but if you propose to evaluate journalistic practices by their contribution to democracy, I think there is a good case against giving prominent and relentless coverage, and against allocating large investigative resources, to material that is a) irresistibly gripping and salacious; b) of negligible intrinsic political importance; and c) likely to drastically reduce the choice of available candidates on trivial grounds. The destructive effect on the space of public discourse of the introduction into it of such material was predictable in advance by anyone with any imagination at all, but now it has been established empirically. The effect is out of all proportion to the importance of the facts even in the estimation of people who think they matter politically.
Ordinary people understand this, but many journalists have a professional interest in being blind to it, apparently. At the time of Gary Hart's exposure, I saw an exchange on television between a reporter and a woman he stopped in the street. He asked her whether Hart's adultery would make it impossible for her to vote for him, and she said yes. He then asked whether she was glad to know about it, and she said no, she wished she hadn't been told. She was not being irrational.