First, no one is calling for a gender ratio of exactly 1:1. Instead, highly unbalanced gender ratios are prima facie evidence of discrimination or some other problem. Say, if things were working as they should, probably we wouldn't see gender ratios of 3:1 or higher; but we do see gender ratios of 3:1 or higher; so probably things aren't working as they should.
Second, there is ample independent evidence, across many professional fields, of serious problems of sexual harassment, discrimination, workplace norms (60-hour work weeks or expectations that workers will be in the office until 8pm or later are incompatible with caring for children or sick family members), implicit bias, and so on. Specifically, in IT and computer software and hardware development, women played a major role in the first few decades of the field, then were explicitly pushed out as computer programming and engineering were transformed into highly-paid professional fields. The argument here is that these kinds of factors are themselves unjust and should be remedied, independent of their exact contribution to high gender ratios. In terms of your post, it's a remote counterfactual to suppose that "each industry is equally welcoming of men and women (no harassment, etc.)."
For some further points, I'm going to paste and lightly edit a short literature review that I recently wrote. I'm short on time, so you'll get academic references rather than hyperlinks.
There's a lot of work in history and philosophy of science looking at the impact of women and feminist scientists in specific fields (as specific as archaeology, primatology, embryology, neuroscience, physics, and so on). Two great collections of these examples are Schiebinger (1999) and Creager et al. (2001).
Briefly, most fields of research have always had a small number of women. Occasionally women have had a major impact on the development of the field, and this impact was lost as canonical histories simply ignored them. A good example is Émilie du Châtelet, who made some key contributions to the concepts of momentum and conservation of energy, but until recently was basically treated as Voltaire's secretary (http://projectvox.org/du-chatelet-1706-1749/).
Then, starting in the 1960s and '70s, many more women began to pursue graduate degrees and academic careers. Many of these women were self-identified feminists, and explicitly or implicitly brought critiques of sexism and androcentrism into their scientific fields. These critiques often identified sexist assumptions and stereotypical metaphors, such as "active" males vs. "passive" females (Beldecos et al. 1988). They also directed more attention to the activities of female animals or human women (Hrdy 2009). In some cases, the drive of redress sexism and androcentrism produced better methods in general. For example, concerns that primatologists were ignoring the activities of female primates led to the development and adoption of more rigorous sampling and observation protocols (see Fedigan's contribution to Creager et al. 2001).
Feminist and women scientists also challenged power structures within science and in the relationship between scientists and publics. Obviously sexual harassment is one example. I believe there's also evidence that women faculty do more work mentoring women students, as well as more diversity-and-inclusion-related service work. (The point that women faculty are often more burdened with service responsibilities then men, and so have less time to spend on primary research and teaching.) Women physicians played a role in the women's health movement (think /Our Bodies, Ourselves/), which was one of the major challenges to the paternalistic model of doctor-patient relationships. Feminist bioethics is a major area of bioethics, which has also changed the way researchers relate to human subjects.
Finally, the experiences of women and feminist scientists have challenged disinterested, impartial, or value-free notions of objectivity and scientific integrity (Longino 1990; Harding 1991; Hicks 2014). Some women scientists have brought a more "caring" perspective to their work, even in fields like genetics (Keller 1984). Feminist critiques have often showed that "objective" science actually involves implicit sexist assumptions (Fine 2010). And feminist scientists have often designed their research to help understand and address important social problems (Hill Collins 2000). In these kinds of cases, bringing in values has actually made science better.
[In the contemporary tech industry, we might consider the fact that many important critics of the social impacts of tech are women: Zeyenp Tufekci, Kate Crawford, Hanna Wallach, Cathy O'Neil, Julia Angwin. I recently spent a year as the kind of in-house ethicist with the National Robotics Initiative. This opportunity was created by a leadership team that was almost entirely women.]
Beldecos, A, S Bailey, S Gilbert, K Hicks, L Kenschaft, N Niemczyk, R Rosenberg, S Schaertel, and A Wedel. 1988. “The Importance of Feminist Critique for Contemporary Cell Biology.” Hypatia: A Journal of Feminist Philosophy 3 (1).
Creager, Angela N. H., Elizabeth Lunbeck, and Londa L. Schiebinger, eds. 2001. Feminism in Twentieth-Century Science, Technology, and Medicine. University of Chicago Press.
Fine, Cordelia. 2010. Delusions of Gender: How Our Minds, Society, and Neurosexism Create Difference. W. W. Norton & Company.
Harding, Sandra G. 1991. Whose Science? Whose Knowledge?: Thinking from Women’s Lives. Ithaca, N.Y: Cornell University Press.
Hicks, Daniel J. 2014. “A New Direction for Science and Values.” Synthese 191 (14):3271–95. https://doi.org/10.1007/s11229-014-0447-9.
Hill Collins, Patricia. 2000. Black Feminist Thought: Knowledge, Consciousness, and the Politics of Empowerment. Rev. 10th anniversary ed. New York: Routledge.
Hrdy, Sarah Blaffer. 2009. The Woman That Never Evolved. Harvard University Press.
Keller, Evelyn Fox. 1984. A Feeling for the Organism, 10th Aniversary Edition: The Life and Work of Barbara McClintock. Macmillan.
Schiebinger, Londa. 1999. Has Feminism Changed Science? Harvard University Press. http://books.google.ca/books?id=Fb232LIFk80C&printsec=frontcover&dq=intitle:has+feminism+changed+science&hl=&cd=1&source=gbs_api.