Professor Mike Ross is a friend of mine from way back. In the late 70s I did my doctoral thesis on how men construe their sexual orientation. Following my research results, Mike and I agreed that labelling and self-labelling were social constructs and only had a moderate correlation with actual behaviour or fantasy.

Basically, I believe that most men are far more flexible than they choose to either admit to themselves or declare to others. If you look at Shere Hite's survey work, she makes the point that a sizable percentage of men who state that they are heterosexual either have or would fantasise about sucking dick or being fucked by a guy though many went on to say that they wouldn't do it in reality because the dick would be attached to another man.

I think that gay porn on the Internet, chatrooms and such bridge the psychological gap between private fantasy and anonymous reality. So my view is that the flexibility has always been there and this is just a newer manifestation.

Bear in mind that historically many men were functionally bisexual. Richard the Lionheart would have thought of himself as straight as would anyone who knew him. But he did have sex with several men, including the King of France and whilst on the Crusades he had his "friend" Blondel. He was both insertee and inserter in anal intercourse.

The categorisation into gay, straight, bi, bi-curious and whatever-flexible are relatively recent distinctions.