You said my analogy works in this case, so I must have shown that it is discrimination based on sex,...
I admit that this is a tricky issue, but these are the analogies that I think work for race with respect to employment:
Black : White :: Homosexual : Heterosexual
Black : White :: Transgender : Cisgender
Black : White :: Woman : Man (Generally speaking, for there are more legitimate employment related distinctions based on sex than on race, but in general men and women should be treated equally with respect to employment. The Court didn't use this analogy because it limps too much.)
But I still don't think that discrimination based on orientation or identity is discrimination based on sex.
...but you're saying it isn't but-for reasoning. So what is the reasoning? I thought I was demonstrating but-for reasoning.
Meh, my OP was probably far too nuanced. Maybe that's why no one replied, but I'm glad you did.
I think but-for-race reasoning works to prohibit discrimination on the basis of race; I think but-for-sex reasoning works to prohibit discrimination on the basis of sex; I think but-for-orientation reasoning works to prohibit discrimination on the basis of orientation; and I think but-for-identity reasoning works to prohibit discrimination on the basis of identity. I don't think but-for-sex reasoning works to prohibit discrimination on the basis of orientation or identity, and I don't think discrimination on the basis of orientation and identity is discrimination on the basis of sex.
My OP is about whether but-for-sex reasoning works to prohibit discrimination on the basis of orientation and identity. It is uncontroversial that but-for-orientation or but-for-identity reasoning would prohibit discrimination on the basis of those categories.
But okay, how would you oppose discrimination based on interracial relationships without but-for analysis?
I think the basic concept of but-for reasoning to identify discrimination is sound even if the legal application is mired with difficult questions. I'm happy with but-for reasoning for identifying racial discrimination.
At the heart of such issues are three simple questions: 1) Is discrimination occurring? 2) If so, what kind of discrimination is occurring? 3) Is that kind of discrimination illegal? Bostock fudged (2) in order to be able to give an affirmative answer to (3) due to the fact that discrimination based on orientation and identity aren't currently illegal.