I think you are not hearing yourself. Let's unpack this: "Trans people are still either men or women, so bisexual people are can theoretically be attracted to them." As their unwanted biological sex, not the one they claim to be but aren't, perhaps, but that is not a blanket statement applicable to all bisexuals by any remote stretch of the imagination and needs to not be promoted as such, which is what you are doing here by claiming that +trans is an orientation, but -trans is a preference.

Being trans is another thing entirely not related to orientation. Understanding and acknowledging the sex one is is sort of baseline requirement for being a candidate for dating. So a trans person, no matter how much I like them otherwise, is automatically not in my dating pool. And nothing whatsoever needed to be modified about the definition of "bisexual" for this to be true, either. The modification is coming from people tacking "trans" on to what is included in the term.

And this is why we deal with trans people trying to bully us by definition. Rewriting the meaning of words doesn't grant the fake new meanings primacy, it just makes people who want power and control look desperate.

Find another term for this. Bisexual is taken. Not sure why you are so desperate to force attraction to trans people to be part of bisexuality in the first place. As we have discussed here many times, "man" and "woman" have clear, understandable meanings related to biology and body parts. If someone wants to play mix'n'match in their heads and then get surgery to feel like they really are that sex now, they are doing something else (and also taking away a key factor that would make them attractive as their biological sex, if being unable to accept that detail about themselves didn't already disqualify them). More power to them, but leave bisexuality out of it. This is as unwelcome and unnecessary (find a more precise term that isn't already taken) of an argument as it would be to say that lesbians should like "girldick." I like my men knowing they are men, and my women knowing they are women. Period.

ETA: Just so it's really super clear, a trans person, even if possessed of an identifiable biological sex and intact body parts, isn't part of the definition of people I'd want to sleep with and partner. So they are not part of the definition of bisexual, or a demographic that would be discussed in a dating context (which is the whole point of a term like "bisexual"), any more than trying to include doorknobs would be relevant—it's not a relevant category of entity. Just like men who insist they are women are not a relevant dating category for lesbians.