It’s oft-repeated but expounded upon. One arena it could be put to the test is with regard to dating preferences.
I'm not shy about asking this exact question among my blue tribe friends. There are a few people, almost always bisexual women, who call my bluff and have no hesitation about dating a transwhatever. But that's a niche situation that isn't representative.
I've commented on trans individuals and dating a few times before. Basically, I'm a cis heterosexual male. Aside from a preference for slight feminine androgyny, I do not find either masculinity or male body features attractive. Dating apps are increasingly annoying to use nowadays because while users do actually label themselves as cis or trans, and while I specifically filter for only cis, I still get shown trans women in my feed and about 10% of my messages are from transwomen. I just think it's a giant waste of time for everyone involved, my preferences are not going to change, but dating apps have sometimes explicitly announced this as a feature for fear of being accused of transphobia.
When I'm swiping through dating apps, it's almost like a spidey sense when something looks "off". After that, I either look in the bio where the trans identity is very often divulged, or I swipe through pictures comparing and contrasting. In terms of instances where someone passes so well that I have trouble believe they are trans, those are less than a handful.
Part of my attraction towards cis women is definitely the mechanics of sexual intercourse which enabled solely because of how body parts fit together. I've encountered the rejoinder that just because your girlfriend has a dick, doesn't mean you have to do anything with it. And also that if you're into each other enough, you'll figure out ways to make the sex work. All true theoretically, but then I think back to times I dated cis women and we broke up because we just did not click sexually for whatever reason. I don't want to roll the dice even further, especially when my interest has already cratered if the only potential outcomes are dick or post-op vagina.
I have an enormous amount of sympathy for trans individuals in how they approach dating. Their pool dramatically and drastically narrows down to a miniscule subset of people willing to date them. I genuinely felt sorry for this transman who recounted how women keep ghosting him. But I just wanted to tell him "well, what do you expect?" It seems like people are attracted to expression as much as body parts. My interest in someone romantically is going to take a fucking nosedive once I find out they don't have a vagina regardless of what other qualities they may have. It changes up the dynamics so much and on such a fundamental level but there is still this delusion that tries to believe body parts shouldn't matter.
As a counterpoint (or rather a ContraPoints...) Natalie Wynn describes her own experience with dating and relays that it's mostly straight (should that be in quotes?) men who approach her for dating. There is always the crude question of "dick or nah?", but otherwise she says men mostly treated her as a woman in almost every way. I'm inclined to accept the narrative of her experience, but at the same time I just wonder "who are these people?" because while I know plenty of bisexual women who sleep with trans folks, I've never met a "straight" man who did. Maybe there is a greater taboo, and so they're far less likely to admit it.
I feel like the definition of "straight" has been drifting from "opposite sex attraction" to "opposite gender attraction" (I believe Andrew Sullivan might have said something about this issue). There is an implied expectation, in line with the mantra "trans women are women", that if you are attracted to women, you are also necessarily attracted to trans women. I'm not sure how people would describe my own preferences. I'm attracted to feminine cis women, I am also attracted to and willing to date pre-op feminine trans men. But that's because I don't really care what people's inner gender identity is so long as they check all the boxes I'm otherwise looking. Caroline Polachek can announce that she now identifies as a man and I'd still totally want to sleep with her. I don't think there is an immediate label for this specific orientation, because acknowledging it would also acknowledge that at least for some people, trans woman aren't really women, and trans men also aren't really men.
For me, I don't categorically say "trans women are NOT women" because I don't believe everyone has the same working definition of "women". So, to set the stage for a normal conversation, I instead just ask "what is a woman?" and you can kind of guess where that goes (hint: nowhere).
While it doesn't matter for most parts of life, if one IS getting naked with a person, then what's in their pants absolutely does matter.