Do straight people exist? https://www.google.com/amp/s/www.unilad.co.uk/life/straight-people-dont-exist-finds-new-study/amp/

Recent studies show that the eyes dilate when people watch male and female porn in all men and women, regardless of their orientation, which indicates sexual arousal.
It turns out all the bisexuals.
But how are studies that say that sexual orientation is not a choice;
Many factors influence sexual orientation, such as: genes, hormones, culture.
Sexual orientation is very stable and is unlikely to change.
Statistics say 90% of the heterosexual population.
How can this be understood?
The article you link to mentions that the sample used in the research was women, there is a passing mention of males having slight pupil dilation when shown images of male masturbation. 

However, the data seems to point to something a bit more complicated, Levay (2017), explains that though women in this area of research typically show stronger signs of bisexuality, even contrary to what they report, men seem to fall into one of two categories, straight or gay, with bisexuality being rarer. It seems that to distinguish bisexual males from straight or gay males requires more careful studies and evaluation, pupil dilation is only one piece of the process.
Levay calls the distribution bimodal between the three categories for males, with straight being the heavier end while gay males are a smaller amount with bisexuality running trough between the former and latter. Levay reports that there are two types of bisexual males, ones that are bisexual as measured by objective measures and have personality factors such as higher sex drive, curiosity and sensation seeking and one that claims to be bisexual but has homosexual arousal patterns. The bisexual sample in one study mentioned seemed to be unaware that their arousal was homosexual, despite claims to be attracted to both sexes. Levay believes that there could be issues of cognitive dissonance in the bisexual sample. Levay suggests another possibility, that there is a lack of feedback due to neural circuits mediating genital arousal to cortical circuits involved in the conscious evaluation of arousal in bisexual males.

So it seems that when it comes to bisexuality in males, there is a greater chance that they may actually be gay but have not fully reconciled any qualms they may have with their sexual preferences. Of course, Levay also mentions that the jury is still out on the issue of dissonance and further research can be done on the topic. This contradicts the information presented in the article you posted and shows that male preference is not as fluid as a female preference.

References:
LeVay, Simon. Gay, Straight, and the Reason Why: The Science of Sexual Orientation. , 2017. Print. https://libgen.pw/item?id=1703978&amp;adblock=true