On The Battlefield Between Sexes
The image is a collage from my work 100 and One Sex(gender) Identities (2016-2017), in which I asked people to playfully define their feminine and masculine sides on a scale of 0-100. I wanted to show how few people define themselves as 100% female or 100% male, even though language and legislation still black-and-white us into the male and female boxes.
Redefining sex
I have set aside the typical notion in academic research of separate biological (sex) and social (gender) sexes in my work. 
I use in the following text the term sex instead of gender because in many languages — like in Finnish, there is only one word for physiological and psychological aspects of it. 
Jemima Repo, lecturer in the politics of gender at Newcastle University, explained in the article Unlearning Gender why the term gender is problematic.
Psychiatric doctors invented the concept of gender in the 1960s when they wanted to let surgeons operate on newborns' genitals with intersex characteristics to look like a typical boy or girl, even when there was no medical justification for the operation.
Medical experts believed that we were born as a 'blank slate' and that parents could nurture anyone to have a male or female identity. This belief has had tragic consequences because doctors have performed on numerous babies of the opposite sex than their identity. 
Regardless of the problematic idea of gender, the feminist movement adopted gender as a political weapon in the 1970s to justify demands for equality; they claimed that there was no biological basis for qualification differences between men and women. 
However, studies have shown that female monkeys prefer doll toys, while male monkeys choose cars or tractors. 
It is a biological fact that sex matters, but because no human is 100% biologically female or male, the shape of genitals is not the destiny of human behaviour.

Self portrait as a male (2006) and as a female (2009)

Sex is a spectrum
The biological development of sex characteristics is not fully understood, but the SRY gene is known to play a significant role in testicular development. 
The SRY gene is usually located in the Y chromosome but sometimes in the X chromosome; male-typical human beings may have XX chromosomes even though it is considered more like female-typical chromosomes.
Medical science also knows male-typical human beings who do not have the SRY gene or have different chromosome pairs than XX or XY (for example, XXY, XXYY, XXXY, or XXXX)

Hijra, oil on fabric, 80x60 cm, 2007

Science also knows of people born with a female-typical body but developed a penis during puberty due to diverged genetics.
The reproductive organs develop from the same organs, which differentiate into female-typical- and male-typical-characteristics, usually after the seventh week of pregnancy.
Sometimes, these characteristics intersect so clearly that doctors can not define the person as a male or a female.
For example, medical science recognises people who have both a penis and a uterus from birth.
An individual may have derived from two or more zygotes, and if the zygotes were of differing sexes, the person might possess both female and male sex organs.
Brian P. Hanley (PhD in Microbiology) writes in his hypothesis, published in Life Sciences (ISSN 20428960), that Dual-gender macrochimeric tissue discordance is predicted to be a significant cause of human homosexuality and transgenderism. 
Hanley presents that (macro)chimerism may be pretty common (between 5% and 15% of people), and this might explain sex identity disorder and homosexual behaviour if the central nervous system develops towards opposite sex-typical traits than a person's gonads.
Sexual variations are a natural part of evolution, even if they might lead to infertility; Evolution happens through variation, and sometimes, these variations lead to superior traits.
Intersexuality is a normal evolutionary variation. Therefore, sex is a spectrum, and we should define sex in terms like feminine and masculine rather than squeeze it into the female/male boxes.
And if we count biotechnology in, every human being presents both sexes because it is possible to transform human stem cells into both types of sex-gametes.
Single-sex reproduction is not possible yet in humans but might be possible in the future because it has been made in the labs with mice already.

 A Horse and a Man (2008), acrylic on paper 120 x 80 cm

ACTIVISM FOR EQUALITY
States determine a person's sex/gender as female or male (or third gender in a few states). However, it is impossible to order the person's sex and juridical status without violating the rule of law because sex is often open to interpretation.
It is possible to study sexual characteristics (chromosomes, genes and physiological characteristics), but the boundary between male and female is open to interpretation because of intersex characteristics.
As an activist, I have tried to influence sex/gender legislation in the Member States of the European Union. – We could achieve, for example, equal marriage laws throughout the EU if the states respected the rule of law and would not continue their privilege to determine the newborn's sexes.
As I see it, the only sustainable way to determine a person's sex is for everyone to select it by themselves.
I asked the EU Commission to assess, in my question (submitted by Sirpa Pietikäinen, MEP), whether the Member States can determine the sex of a citizen at all since it is recognised in medicine that there is no flawless test for deciding it. However, my question did not lead to the result I had hoped. – The Commission did not answer clearly on my question.
Towards a healthier definition of sex
Defining sex as a spectrum instead of a compartment would also pave the way for a healthier definition of sex identity.
Many people struggle with the pain of not being a perfect woman or man. However, it is impossible to be an ideal female or male because no one is biologically 100% of either.
Defining sex by adjectives (such as a female human, a male human instead of a female, a male.) would be a more biologically accurate expression. This would also open the way to narrow sex boxes.
Instead of sex/gender boxes, I usually define myself as a feminine person with distinctly feminine and masculine physiological traits. In this way, I feel freer from women's sex/gender expectations.

 Shemale Namu, 2020, piirros paperille 30x50 cm

Redefining sexuality
If we define sex as a spectrum, it will break down the definitions of gay, lesbian, and heterosexual identities; those were built on the idea of binary sex.
Scientific studies have shown that people respond more or less to videos of both male- and female-bodied masturbating, even if they identify themselves as 100% heterosexual, gay or lesbian. – It is obvious from a biological point of view because the boundary between males and females is fluid. Therefore sexuality cannot be defined by sex boxes.
Finnish PhD researcher Jarkko Oraharju has considered whether we could define sexuality as a fetish. I agree that we are always attracted to traits rather than sex/gender boxes. Therefore, it would be more meaningful to define sexuality as an interest in femininity, masculinity, body parts, or any features rather than divided physiological sex.
I explored this topic in practice with the experimental dating app Pandate (2017-2018). This online dating app allowed users to define their sex and sexuality on a sliding scale.
It was particularly appreciated by those who defined their gender in a non-binary way. However, the work unexpectedly attracted criticism from many people with gay and lesbian identities, who could not accept the decontextualisation of sexuality.

Gay (2005), oil painting 150 x 80 cm kankaalle.

My publications on sex and sexuality
(mostly in Finnish only)
Valtio ei voi määrätä sukupuoltani – QX, 2018
100 ja yksi sukupuolisuutta, a community artwork about defining sex/gender on a scale of 0-100 femininity and masculinity. Published August 2017- 2018.
PANDATE – OUT-OF-THE-BOX DATING. An experimental dating app without sex/gender and sexuality boxes. Published as a web app in 2017-2018.
ESSEITÄ SUKUPUOLESTA 1, Sukupuolesta sukupuolisuuteen, Susa Tyrniluoto, Bohem Print (self-published), February 2017.
Sukupuolilokeroiden illuusio - Koneen Säätiö : Kone Foundation blog publication, 2017.
Sukupuolten Tulilinjalla, lecture at the Pori Unit of the University of Turku, Gender Studies course, 22 November 2017.
Gender determination and the rule of law, Questions from Members of Parliament, 25 October 2019: https://www.europarl.europa.eu/doceo/document/E-9-2019-003458_FI.html. Commission's reply: https://www.europarl.europa.eu/ doceo/document/E-9-2019-003458-ASW_FI.html
Sources
Unlearning Gender, The Philosopher, vol. 107, no. 3, Jemina Repo, Lecturer in Gender and Politics at the University of Newcastle and Docent at the University of Helsinki: https://www.thephilosopher1923.org/repo.
Sex differences in rhesus Monkey toy Preferences parallel those of children, Hormones and behavior vol 54.3 (2008), Hassett, Janice M et al., Https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2583786.
How is biological sex determined ?, Studia Medicina, Chief Physician and Professor of Developmental Biology Hannu Sariola, (HY, HUS), Biology and Psychology of sex/gender, 16 January 2019.
A case report of an incidental finding of a 46, XX, SRY-negative male with masculine phenotype during standard fertility workup with review of the literature and proposed immediate and long-term management guidance, Fertility and Sterility, Volume 99, Issue 5, 2013 , Neil A.J. Ryan, Shahnaz Akbar, https://pubmed.ncbi.nlm.nih.gov/23290744/
Beyond the Sex Binary: Toward the Inclusive Anatomical Sciences Education. Anatomical Sciences Education, Strkalj, Goran & Pather, Nalini. (2020), https://pubmed.ncbi.nlm.nih.gov/32735387/.
Sex redefined, Nature 518, 288–291 (2015), Claire Ainsworth, https://www.nature.com/articles/518288a
Sexual Orientation: Categories or Continuum? Commentary on Bailey et al. (2016), Ritch C. Savin-Williams, https://doi.org/10.1177/1529100616637618.
Prosociality and a Sociosexual Hypothesis for the Evolution of Same-Sex Attraction in Humans. Front. Psychol, Barron AB and Hare B (2020), https://www.frontiersin.org/article/10.3389/fpsyg.2019.02955.
Robust evidence for bisexual orientation among men, Proceedings of the National Academy of Sciences of the United States of America vol. 117,31 (2020), Jeremy Jabbour et al., https://pubmed.ncbi.nlm.nih.gov/ 32690672.
US proposal for defining gender has no basis in science, Nature, 2018, https://www.nature.com/articles/ d41586-018-07238-8.

See also: