Sorry, got backlogged on stuff...
... She's still switching her definition half way through because now she's talking about secondary sex traits. By any definition, secondary traits are not part of "whether the person has the small gamete machinery or the large gamete machinery" - it's how we decide that they are "secondary".
I don't think she is. I think she's making a distinction between the traits people use to
define sex and the traits we use pragmatically to
assess sex; I think it looks like a switch to you because you aren't picking up on that distinction. But I'd best let her speak for herself on subtleties like this.
Well she did say this, in the post I was replying to: "What are the odds that you - as a heterosexual male - would meet someone, like them, and then be *surprised* to find that they have a perfectly typical male anatomy with perfectly typical male primary and secondary sex characteristics?"
That question is irrelevant to the issue we are currently discussing, unless she switched the definition. She claims that sexual orientation sorts by biological sex alone, and insists biological sex is determined fully by primary sex traits. I claim that it is primarily secondary sex traits. The scenario where the two correlate as they typically do is useless to distinguish the two hypotheses as they make the same prediction here.
She said that in response to your statement, "I can only speak for myself, but while self-ascribed gender isn't a factor in what makes me attracted to a person initially, neither is what you call biological sex, i.e. the type of gonads a person has, or even their external genitals, and for the same reason too: those are things I do not see." What you'd said was irrelevant for exactly the same reason you claim what she said was irrelevant -- the two hypotheses make the same prediction here -- and she was pointing that out.
The only way she can claim an answer of "the odds are slim, obviously" to the above question as a win for her theory that sexual attraction is driven by biological sex alone if that now includes secondary sex traits.
The so-called "win" for her theory is simply that your "those are things I do not see" argument is not a lose for her theory. So what her question achieved was really more a draw than a win -- decisive evidence will need to be sought elsewhere -- but blocking an opponent's goal shot is no more irrelevant than scoring a goal of one's own.
If biological sex is the sum of primary and secondary traits, she can no longer claim that ... or that trans women are "just men" who pretend to be women.
Where the heck did you see her claim that? Where did she in any way imply men who say they're women are "pretending"? What, are Christians "pretending" there's a god? Most of them are perfectly sincere. Humans have been known to be mistaken from time to time.
Sure if the question were why evolution produces heterosexual attraction as the majority behavioral phenotype, the answer would be that the close correlation between primary and secondary characteristics makes the latter a good proxy for the former and this enables individuals to hone in on potentially compatible mates. But that is a diachronic-functional perspective, while the question what triggers it is a synchronic-procedural one. If there is just one lesson behavioral biology had learnt since the mid-20th century that we should keep in mind at all times when discussing the topic, it's that those two types of questions need to be distinguished and tend to have different answers. In Niko Tinbergen's terms she's presenting the answer to question 1 as the solution of question 3:
https://www.conted.ox.ac.uk/courses/samples/animal-behaviour-an-introduction-online/index.html
I'm not seeing that. It looks to me like she's presenting a straight-up question 3 answer, just one that's different from yours. You appear to be systematically failing to distinguish between the traits people use to
define sex and the traits we use pragmatically to
assess sex. That's why the place to look for evidence is the question she asked at the end: "The real question is whether or not you would continue to be physically attracted to them when they drop their pants and you see their genitals. As a straight man... how likely do you think you would be to enter into a sexual relationship with someone who has a penis?" So let's look at your answer.
"Well, this one heterosexual male here disagrees. I'm attracted to the whole package, of which the vulva is one part I typically haven't even seen when something in me decides I am attracted to that person. If there is no vulva, it makes matters more complicated, but how is it going to magically change the fact that I'm already attracted to that person? We can do ass stuff. We can switch up - I've done strap-on stuff and liked it. That's probably not going to be my first choice for a longterm relationship, but that's about it."
It's possible this is just me Yanksplaining and things are different in Austria, but what you describe is not a typical heterosexual male reaction. For most of us if there is no vulva that will
nonmagically change the fact that we're already attracted to that person.