Hey, I've been following this exchange with s/wafflegaff, and while I certainly wouldn't presume to speak for her, I thought that maybe my own take might be helpful here.

Then please tell me what is the sexuality of people dating TWs or TMs?

Here's the thing: I don't think that such people HAVE a single sexuality, in terms of orientation. They are, respectively, male-attracted and female-attracted. Yes, "bisexual" encompasses both, but what's important to remember is: we bi people aren't any more likely to be attracted to TWs than are straight women or gay men, or TMs than are straight men or lesbians. So, essentially, those of ALL sexual orientations-- homosexual, heterosexual, and bisexual-- are functionally the same in this respect: we're attracted to biological sex. And since this never changes for trans people, NONE of us can be attracted to TWs as anything other than the men they are, or TMs as anything other than the women they are.

If there is a person who likes both sets of primary and secondary characteristics, set the right way on biological male and female, but also happens to like them mixed up, what is it then? It's bisexuality with a trans fetish.

A few things occur to me here. First, trans people don't have "mixed up" primary and secondary sex characteristics; they have the ones for their biological sex. The only people for whom this isn't true are those with rare forms of DSD (aka intersex conditions), and no amount of medical intervention will give anyone that. So what some trans people have are artificially-induced features meant to mimic the primary and secondary sex characteristics of the opposite sex. These usually strike observers-- whatever their sexual orientation-- as patently fake, in part because they don't erase the natural ones, which remain evident and to which our entire species is finely-attuned.

The upshot of all this being: fetishism isn't required to explain why bisexuals could potentially be attracted to both TW and TM (although we are no more likely to be attracted to TW than other male-attracted people, or TM than other female-attracted people)-- in fact, this is much more applicable to monosexuals than to us. For example, if a straight man is attracted to a TW... well, what else could possibly be going on there? Same thing with a lesbian attracted to a TW, or a gay man attracted to a TM (assuming, of course, that all of these people are aware of the trans person's actual sex).

I'd also like to add that this sort of talk sets off alarm bells for me, and probably many other bisexuals. Specifically, that we're inherently inclined to be attracted to trans people. Yeah, I know that you aren't positing precisely this; since it's such a pervasive belief, though, anything along those lines risks being mistaken for it, and thereby adding fuel to the fire. Given that we and our sexuality are seen as suspect anyway (in some ways even more by gay people than straight people), this is something bisexuals REALLY do not need.