the website is funky, so I can't seem to quote tonight... This is a reply to Lusita.
I think that you are overlooking the time factor that I was highlighting: a 16 to 17 year old over-enthousiastic boy new to the game, and a 20 year old pop star who has gone through a lot more experiences (some pretty bad ones too I'm pretty sure in regard to the press & fan scrutiny) and has grown up are not exactly the same people IMO.
Both the Matt Cardle thing and the hook up with fans rumours (and the phone call with the fans that you linked) happened quite some time ago, when everything was still new to them.
And I have no problem with Harry keeping it cleaner or not wanting to talk about whether is is an ass or boob man. If he realises that his mom or dad or grand parents (or younger kids even) might watch the interviews, and it leads him to a certain behaviour & it's fine with me. I suppose to certain people it might seem disingenous. To me it just seems like someone who wants to keep more things private than the fans would like (which I have no issue with) & someone who realises just how far his words can reach.
I personally find some of those PC answers very charming, especially the way he delivers them, but I understand that some might not

I think Harry gives an insight into the kind of person he is (he is a nice guy, he isn't confrontational but he seems smart, patient & able to deal with pretty tough situations). However, he doesn't want to share certain things, either because, like you think, he is too media trained, or because, like I think, he doesn't want to over-share (for whatever reason).
Harry is almost 20 & has a private life that I don't think we know anything about (we didn't even realise that he was "homeless" & crashing at friends untill Spurlock mentioned it).
Since the problem seems to be his private life and the fact that he doesn't chose to share anything about it: maybe he hooks up with many girls based on their looks solely, maybe he just has regular sex-friends based on their looks (and personality?), maybe he really is a lot less active than we imagine... Fact is we don't know & we assume, just like the gossip outlets.
And I find it unfair that Niall's happy-go-lucky "I don't have any GF and haven't for 3 years" isn't probed quite as much by fans as Harry's private life. But like ImperfectPearl said, Niall seems to be in this bubble, and the fans seem to somehow think he can do practically no wrong. Not that I think he is doing anything wrong BTW, but the difference in expectations about being forthcoming about their private lives irks me.