 A third group of fans will claim it's all because the actors in question were hungover at the time and off their game, and it's only the dialogue in the scene that matters.  A fourth group may see the dialogue as being a carefully-constructed set of mutual lies, based on the obvious truth that both characters were being mind-controlled at the time by aliens.  Fanon is fun.  Also headache-inducing, especially when confronting a True Believer on any particular piece of said fanon.

Your fanon is your fanon.  My fanon is my fanon.  And that's okay.  Parts of your fanon may be jossed by later canon.  Parts of my fanon may be jossed by later canon.  It happens.  Blaise Zabini is not a Caucasian girl.  Terry McGinnis is not Bruce Wayne's clone.  And so on.  Accept the new canon, go AU, pretend nothing exists past a certain point in the canon (a personal favorite tactic of mine), whatever gets you through your day.  Just remember that other fans are doing the same thing in their own ways, accepting or dismissing the new canon as needed for their own sanities. Going into their sandboxes to kick up a storm and whiz on their toys because their fanon is not your fanon is counterproductive and rude, and will only lead to wankstorms.

Fanon that is accepted by the majority of the fandom is still fanon.  If your fandom's fanon claims that members of Species X mate for life, good for you.  Unless it's in the canon, that's still fanon, no matter how widely-accepted the fanon might be.  If your fandom's fanon says a minor character has a particular name, until and unless the name makes it into canon (such as Nyota Uhura and Hikaru Sulu, both of whom were not officially given first names until the movies, and the former not until this year), that character's name is fanonical.  Figwit the Elf, so named by LotR fans who watched the Council of Elrond and said, "Frodo Is Great … Who Is That?" is unnamed in canon and may be one of Elrond's sons, or one of his attendants, but whom the producers started referring to as Figwit http://www.figwitlives.net/dvdcom.txt because of the fanon funny.

So if your fanon isn't canon, what does that mean?  Nothing, really.  We're fans.  We're used to creating our own realities within the canon realities.  Most slash is fanon.  Much  het is fanon.  All of fanfic is based on or creates new fanon.  We can argue about it, but since we're all stuck with the same canon (whether we want to acknowledge it or not) there's not much point in flaming someone for not playing along with your particular version of fanon.  Go play somewhere else, set up your own sandbox, and hang out with other fans who think Dumbledore and McGonagal were truly and deeply in love and that Tonks was just a rebound for Remus after Sirius died. 