A new study from Gerulf Rieger and Ritch Savin-Williams confirms the relationship between sexual desire and pupil dilation. While this relationship has been known for some time, now it is possible to measure the dilation with precision. In doing so, the authors report significant relationships between self-reported sexual orientation and the way pupils dilate in response to sexual imagery.
The authors report that males are more clear in their preferences than women, meaning that whether gay or straight, the dilation of their pupils matched closely their stated preferences. Women were more likely than men to show pupil dilation to both preferred and non-preferred images. Bisexuals also showed dilation to both male and female sexual images.
The results are not perfect in that there is variability of pupil response within sexual orientation groups. These results say nothing about causes of orientation but they do provide more evidence that the attraction component of sexual orientation is mostly involuntary.