“This changes the relatively smooth bell curve of the past data to one with an even greater spike at the 50/50 mark, showing, at least using this same sample population, an even greater move towards versatility since I crunched the numbers back in 2006.”

You’re using self-identification for the data set rather than independent observation. I’d suggest the change may be related to early education that “we’re all the same” and “everyone is equal,” rather than a change in sexual practice or preference. The surveyed population may have a greater proportion of people now than in the past who are afraid to admit that they actually like some things more than they like other things. My “evidence” is anecdotal or less; more research is needed. (But keep in mind that my evidence is just as good as your evidence, because all evidence is equal, all points of view are valid, and everyone is equal. Ahem.)