In The Journal of Personality and Social Psychology, researchers studied several straight men and women watching straight and homosexual adult films.
The purpose is to find out if the people watching these films were physiologically influenced by excitation according to their sexual orientation, by observing their pupil dilatation.
After several days of observation, this study showed that women's pupils dilated when they watched a man and a woman making love, but more surprisingly, also when watching two women doing it. The same phenomenon was also observed in men.
‘When we show a straight men the picture of a woman masturbating, we see that there is an excitement physiologically, but then when we show them a man who masturbates, their eyes dilate as well. We are able to demonstrate that physiologically no one can identify as gay, straight, or bisexual.’ says Ritch C Savin-Williams, the author of this study at Cornell University.
In other words, our sexual orientations are influenced by social norms, not by biology.