Whedon? Really? Well, the writers at Marvel have some REALLY bizaare competitive relationships IMO. One article I read with Millar and Loeb, they totally ripped each others storytelling, while at the same time were being really friendly.
Well, I've read Whedon say plenty of times how Gwen would've never slept with Norman. Actually, I remember the actual quote. As Astonishing X-Men was being released, he was talking about writing such established characters, and how to still surprise them and make them fresh. It was a total X thing, but all of a sudden in the middle, he says "but there's always certain things that you now characters would never do. For example, Gwen Stacy would never sleep with Norman Osborn. Never." And then we have him clearly rip the piss out of Sins Past in Stan Lee meets Spider-Man. I thought it was hilarious too...
Well, to be honest, Gwen sleeping with Norman had nothing to do with JMS. We all now know that JMS finished the story, and Deodato did a lot of the character designs, (and a fair amount of art,) with Peter as the father. At the last second, Joey Q forced it to be that Gwen slept with Norman. Something like a year after the story, Joey Q has taken full responsiblity and has admitted that it was all him now anyway...