hmm....personally, I've never really doubted that Luffy and Ace are brothers. They say they're brothers, and that's all I really need for proof.
There could be many reasons for them having different family names~ and many interesting ideas have already been posted here on that ^^ The most likely to me is that Dragon is not Ace's real father, but since Garp says he trained both of the boys, I think it's equally possible that Ace just decided to change it. ANBU4U brings up a good point about him wanting to change it due to his famous, and depending on your perspective, infamous, relatives.
But even if they're half-brothers or not even related by blood at all (which I doubt, because to me they look very similar), as long as they consider each other to be brothers, then they are, imho. So in the end, having different family names or the degree to which they are related by blood doesn't really mean a whole lot to me, I suppose ^^
on a side note, to the best of my knowledge Portgas does not spell Monkey backwards, so I think this theory can be put to rest
They act the same, have the same D qualities, act brotherly, and consider each other brothers without thinking or showing otherwise. Ace also looks like Luffy, so I don't see the problem there. And I heard somewhere that "Portgas" was a nick-name of Ace's so I dunno if you can count that as a separate surname.
Turns out, they aren't blood-related
Those this really proves they actually are not blood related though? It does prove beyond doubt they are not brothers but it does not prove there is no connection whatsoever between luffy and roger. It also does not explain why luffy actually somewhat resembles roger and ace(specially ace)
they didn't look as brothers (I mean by the appearances)
, well, if you know the true, you begin to find/search the points of difference
I even find Ace more like Dragon than Roger
the same for luffy (more like Roger than Dragon)
cuz of the face and more important : the smile
I know it sound crazy but anyway
And definitely they are related.
i think it might be possible that ace is adopted since he doesnt have the monkey in his name.