>I have very
>little knowledge of the source,
The source doesn't support Utena being any degree of asexual, but it does give plenty of reasons for her to be skittish about physical and emotional intimacy, and to have some trust issues.
>but I can't recall anything from SotS
>to disprove the idea that she's mostly asexual.
In the text itself, I don't think there's much grounds to claim that she's asexual specifically, as opposed to the trust and intimacy issues above. Certainly, what I recall of the Symphony material that addressed her history on Cephiro (pre-Symphony), seem to support that interpretation, rather than asexuality.
>case, philia is prerequisite to eros,
The term for that is demisexual. It's a relatively new term, as far as I know, and has emerged because, like "pansexual" as opposed to "bisexual," or the -romantic suffix (e.g. heteroromantic, homoromantic) in addition to the -sexual suffix, human sexuality is really really complicated and not remotely binary (even on multiple axes). (And that doesn't even touch on gender identity, which is no less complex but only tangentially related to sexual and romantic attraction and desire.)
>and she doesn't
>seem to have much of anything in the "base lust" department.
See above about the intimacy issues; while it's been some time since I last read the stuff from before I gave up on Symphony, I got the impression that her recalcitrance had more to do with the way she was used and abused by Akio and Kouga (as I recall, though it's been some time since I watched RGU in its entirety) and, well, Anthy still being out there.
>case (3 or x), her claims of being almost 0 could easily owe more to a
>heteronormative cultural background than her own feelings.
This is a bit of dissonance between RGU and SotS; in the anime, as I recall, being bi- or pansexual was the rule; the show was not remotely ambiguous about Kouga and Akio, and only somewhat ambiguous about those two and Saionji. Similarly, girls crushing on other girls (if not much more) was pretty routine -- Utena and Juri, IIRC, both had substantial female fan clubs. (There were a few people who had only a single target of attraction, at least functionally speaking -- Nanami's fixation on Kouga, for example -- but that doesn't invalidate the general impression. Nor does Miki's apparent asexuality.) SotS alluding to Cephiro having some degree of baseline homophobia in the cultural background is a bit at odds with that.
Some of the RGU bit can be explained by Japanese cultural attitudes towards adolescent sexuality, in that "experimenting" with the same sex (as we'd call it here in the West) is an accepted and normal part of growing up. (This is more true for girls than boys, mind.)
"Mathematics brought rigor to economics. Unfortunately, it also brought mortis."
- Kenneth Boulding