Except for ad hominem attacks, this is perhaps the best-known persuasion no-no. You can't just say, this idea isn't good, because other ideas, now suddenly within reach, are bad. Where do you draw a line? Somewhere. Of course, we all know this, and the lazy application of this line of reasoning is so prominent that, even on the wikipedia page for it, the laziest possible use dominates the extremely important discussion about complex consequences.
Really, we shouldn't be so dismissive of complex consequences, but I don't think that's the core issue in these discussions. When people say it would be a slippery slope to accept and defer to transsexual identities, they aren't actually concerned (most of the time, I think) about their own judgment and sense of reality eventually being wholly subverted by the rigors of turbo-politeness. I'd be hard-pressed to believe they are making that claim – it'd be simple enough to avoid that particular slippery slope's end.
I think the complaint is the following (with the massive caveat that I'm attempting to guess the thinking behind an ideologically diverse set of people – some fraction of which just dislike trans people or think they're far too gross to engage with – and obviously this won't apply to all of them, or any at all, and shouldn't be taken as a statement about any single person's beliefs): 'If we change our culture in this way, to accept people previously shunned, we'd be doing so before we know for a certainty it would be a good idea. That certainty would take a long time – modern American culture is so weird and unprecedented, and faces uniquely weird challenges, that any change could disrupt the fragile, strange, majestic thing we've built.
'And the spooky part is, it's pretty evidently going to be completely fine. But that's precisely the wrong lesson to learn! The difference between the Enlightenment and Pol Pot is relatively small, in the span of human history and thought, considering any major organizations at all are so deeply unnatural as to have typically required worship. We really ought not to mess with success, and messing with it and having it go well makes us even more willing to mess with it in the future – that much has been proven by the success of the campaign for gay marriage. It was hardly a single breath between making a change that big and advocating for trans rights – but we didn't even wait for the results yet! Surely, no matter how much a slam dunk excellent idea e.g. gay marriage is, if you value civilization, surely we should at least wait and make sure massive changes don't goof with anything important.'
This is what I imagine when I hear someone is "a traditional conservative". I've not heard anyone say this aloud – even acknowledging the obvious, that nobody can see any real problem with e.g. gay marriage, would be self-defeating. I wouldn't require that level of candor from a stranger, or the level of introspection to grasp the core ideology. But the emotional sense, that things are good and you ought not mess with important things, I think we all feel that.
Hopefully we can have better conversations about it, because the stuff I'm hearing out there is pretty sad.