I found this article claims:
74 percent of women who identified as “straight” were strongly sexually aroused by both men and women
I found another claim from Telegraph which is most famous UK online version of the newspaper.
A study has found that most women who say they are straight are in fact aroused by videos of both naked men and naked women.
Results along these lines have been replicated in multiple studies. Here are quotes from the abstracts of a few relevant articles. Note that most but not all of these studies list Gerulf Rieger as an author.
Published in Psychological Science, 2004:
Both heterosexual and homosexual women experienced strong genital arousal to both male and female sexual stimuli.
Published in Biological Psychology, 2005:
Women showed [...] large increases in genital arousal to both human male and female stimuli.
Published in PLoS ONE, 2012:
Among men, substantial dilation to both sexes was most common in bisexual-identified men. In contrast, among women, substantial dilation to both sexes was most common in heterosexual-identified women.
However, these studies should be viewed in the wider context of the literature. For an overview of that research, here is a good place to start. To quote an especially relevant paragraph, note that the following seems to basically contradict the findings above:
There is evidence of a gender difference in responses to specific sexual content. Still photographs of nude or partially clothed women or men do not generate either self-reported or genital sexual arousal in heterosexual women (Laan & Everaerd, 1995a, 1995b), but are sufficient to generate substantial subjective and genital responses in heterosexual men (Tollison, Adams, & Tollison, 1979). For men, depictions of affectionate, nonexplicit interactions (e.g., cuddling, kissing) between clothed women and men significantly increase subjective and genital responses but, for women, both significant arousal responses and null effects to these same stimuli have been reported (Suschinsky, Lalumière, & Chivers, 2009; Wincze, Venditti, Barlow, & Mavissakalian, 1980). More recently, we have found that both heterosexual and homosexual men, and homosexual women but not heterosexual women, showed genital responses to film depictions of their preferred sex engaged in nude, nonsexual activities, such as walking on the beach (Chivers, Seto, & Blanchard, 2007).
