There have been lots of interesting responses to the original post, that's for sure. It got me to thinking about exactly what was being tested & measured, especially via penile response. One might say that what turns you on is an indicator of how you feel about your sexuality but I don't think it would include the fantasy vs. reality factor.
If I were to see a very attractive female, I may be very well turned on by her image. If, later I discovered that she had that little something extra, I'm sure that the attraction would diminish considerably. Then there are things that I may or may not think about that might be stimulating that I'd not take part in in reality. So I'm not sure if that would tell the whole story... of if anything could tell it.
I'd think that would be the same thing if one were to judge one's likelihood to cheat on his/her spouse/significant other by checking their reaction to heterosexual porn. A lot of people would react to watching various porn scenes of fantasies that they may have but that doesn't mean that they would actually take part in those same things even if they had the chance.
Me? Not much interest in anything or anyone other than the lady I've shared my life with for all of these years.
Lacy PJs