i favor that one because:
it accounts for most intersex variations. many intersex people get left out in radical feminist conversations about sex based oppression because it is assumed that intersex people are just "not common enough" or don't face female sex based oppression. although, to be clear, i wouldn't claim to be familiar with every single known intersex variation. however, as someone who has studied genetic conditions extensively, intersex people still do align within the bimodal model of sex (except for a few exceptions i will elaborate on), it is their sex characteristics that do not.
it also accounts for all kinds of females, no matter their genitalia, chromosomal arrangements, or anything outside the norm of XX. for example, someone with Turner syndrome is still undoubtedly of the female sex despite only having one X chromosome and is still oppressed based on their female sex characteristics - people with Turner syndrome are also oppressed on the basis of being intersex. another example is a female person who has gotten a hysterectomy - they are still female despite not having a uterus.
the "female = XX" argument is regressive and also factually incorrect, as well as "female = having a uterus/vagina/vulva/etc" and "female = the capability to become pregnant." all of those definitions leave out many females. "female = inactive SRY gene" includes all females, including intersex ones, who are intersex but still face female sex oppression as well as intersex oppression. no argument that involves the use of XX/XY chromosomes to decide who is and who isn't a female is correct because it isn't just the presence of the Y chromosome that determines the female sex, it is specifically the SRY gene, which determines male sex development. a female can have a Y chromosome. this is not pseudoscience or "trans ideology" - as some TERFs would say) - as long as the SRY gene is inactive, a female can have any arrangement of chromosomes, genitalia, etc, and still be female.
the SRY gene is either present or not in most intersex conditions, except in cases of X0/XY mosiacism (1 in 15,000 births) in which some cells have the SRY gene and others do not, which leads to variable outcomes (from phenotypic male to Turner syndrome female). obviously, that means that the definition is not the sole definition of what it means to be a female, it is simply the most encompassing one. if radical feminism wants to fight for all females, then it is natural to take the widest definition, even if maybe 1 in 15,000 of those people have an ambiguous sex. radical feminism is first and foremost a sociological movement, not a biological one. if we are talking about sex based oppression, there are some people who are similarly oppressed as the phenotypically normal female, but are not classified as a typical female or even a female at all. that doesn't mean we shouldn't fight for them, it means that radical feminism should take the loosest definition of female because really, what person is checking their chromosomes before oppressing another person? the gender identity of "woman" is imposed on anyone who is perceived to be a female, which usually involves people who do fulfill my definition, but occasionally includes people who do not. that doesn't mean those people don't face sex based oppression.
if you are phenotypically female, even if you have ambiguous genitalia, even if you have a Y chromosome, even if you might not be female at all, you are still oppressed by anti-female oppression. if that sounds like nonsense - it is like a feminine straight boy being oppressed by homophobia. he is not gay, but because he exhibits traits that society views as homosexual, he is still oppressed by homophobia. it is a similar concept - if a mosaic Turner syndrome female has a SRY gene, that doesn't mean they suddenly are not oppressed or that they aren't medically and sociologically considered a female. i mean, in my research, all people Turner syndrome are called females by the medical community - again, this does mean that the SRY gene argument is still somehow flawed, but i would rather take "1 in 15,000 might not fit this definition perfectly" rather than the XX chromosome argument or the uterus argument, in which billions of females would suddenly not be considered female anymore, due to a really regressive and only basic understanding of biology
you also might be wondering, "why use the SRY gene argument if you admit that some people who do have the SRY gene are considered medically female?" - again, radical feminism isn't a bunch of biologists in coats determining exactly which individuals are female and which are not. we focus on sex based oppression, but even sex based oppression isn't entirely based on sex, per se. many people think that CAIS females are male because they have XY chromosomes, yet CAIS females are still oppressed for having female sex characteristics. granted, CAIS females don't have an active SRY gene, which means they do actually fit my definition of female, but my point still stands that taking in account for intersex variations, sex based oppression isn't cut and dry. endlessly trying to argue what a "woman" is (again, woman is a gender identity) completely ignores sex based oppression. endlessly arguing if an intersex person is truly "female" is also ignorant and meaningless.
the reason i don't define female = female sex characteristics, btw, is because there is no concise definition on what those really are and you could lose those characteristics over time (or not have them at all) and still be considered female. it would exclude females with ambiguous genitalia, who are still oppressed for being female, but don't have all sex characteristics of a phenotypically normal female.
the arguments above aren't to say that females are oppressed because they have all have female sex characteristics. granted, not many radical feminists would consider an X0/XY phenotypically normal male to be a female, but if they experience female sex oppression, then i don't really care if they "look" female or even identify as being female. this is also why i generally consider the arguments to be stupid - mostly because they are arguing "what is a woman?" when woman is a gender identity - what they really mean is "what is a female?" which is an equally complicated answer but has some basis in actual biology, plus, no trans woman is claiming to actually be a female, so i consider it very stupid to relentlessly ask trans people what a woman is when "woman" is entirely made up, and rather, we do mean female, and no trans people are claiming to be able to change their sex, which imo we should leave trans people out of that argument entirely
i also recognize that i use "inactive SRY gene" and "absent SRY gene" interchangeably, when those are different phenomena. in general, i consider both those who have a SRY gene (but it is not active, and therefore, no male sex development occurs) and those with an absent SRY gene to be female. i suppose you can also argue that "female = lack of male sex development" but that does leave out quite a few intersex people who are technically female, and you would have to say how far in male sex development until someone "becomes male" and is no longer a female, a task that is tedious, difficult, and frankly unnecessary to do. it's also a catch 22 because it's just saying "female = anyone who isn't male" which isn't actually a definition
