A lot of really good and interesting discussion here. If I may, however, I'd like to add a further problem with the conflation of trans* and sexuality issues in terms of how they're discussed as 'fixed' or otherwise, alongside the earlier point raised that someone having an established 'sexuality' as part of their identity is actually a really modern concept.
(And on that point, Anc. Greek and Roman men did not in any way consider themselves as having a 'fixed' sexuality which was something that they were stuck with, for them it was a case of when it was socially appropriate to be sleeping with men/boys, vs. women, and there was no idea that they would be primarily attracted to one sex or another - certainly most evidence we have suggests the idea that [all] men would be attracted to both. The case for women is more complicated, partly by lack of evidence of sexual agency at all. I am therefore somewhat dubious about the idea that we've only just become modern enough to accept the long-standing truth that everyone has a fixed, natural, biologically caused sexuality which previously (e.g. in Victorian Britain) they just didn't acknowledge - even in much more sexually permissive societies (as regards this aspect, anyway) this hasn't necessarily been the case.)
The point I actually wanted to ask about is what these arguments of 'fixed' sexuality determined before birth, and thus apparently similar to an internal sense of what gender someone is, do about bisexuality, or the reasonably mainstream idea that it may be more appropriate to consider sexuality on a spectrum rather than 2/3 fixed points. There are two problems here which I think would make sexuality a problematic analogy for how 'gender' is 'biologically fixed' - the first being that bisexuality exists at all, so even if you insist that everyone is 'fixed' biologically into one of 3 rigid, determined categories, does that not cause you some concern about the idea that there's apparently no correspondingly middle position in your ideas of being biologically 'fixed' as male/female in gender? (This is far too often ignored as something to be considered, and I wonder how often bi-erasure comes in as a limiting factor here, sadly).
Then, my second problem - if you allow at all for the idea that sexuality can be fluid, and change as someone's life goes on, and they experience different things (and indeed, are socialised differently!), even if it may have a determined trend in one direction or another which doesn't change, does that not also make it a terrible comparison for some sort of 'fixed' internal idea of your gender? Would you not also have to allow for more fluidity in this internal idea of what gender someone is, in order to say that they are both formed similarly by some biological root?
Or do you think that this is also impossible for sexuality, since effectively x amount of hormones (or similar) in the womb results solely in 'a' sexuality, y amount of hormones in 'b' sexuality, and thus the same happens with 'male' vs. 'female' 'biological gendering'?
I am not sure how coherent this is - just some ideas I've been mulling over, really, but I'd be very grateful for anyone who has thoughts. Even if I weren't rather firmly fixed on the radfem approach to gender and gender identity, I still think there's some stumbling blocks to work out on these biological ideas of such complicated things as gender and sexuality, that at the very least should make us wary of lumping them together the way some do!