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 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.