The main thing I have against Tieflings or part-Demons or whatever, along with Dragonborn/half-Dragons, Drow-as-PCs, and all the rest of those "monster" races being baked into the game as PC-playable is that those sort of creatures are supposed to be what the PCs go out and fight! I don't mind an occasional rare exception for RP reasons or whatever, but when I see a party lineup consisting racially of a couple of Tieflings, a Dragonborn and a Goliath my first response is "why is everyone playing monsters?".
Lanefan
Honestly, a serious problem here is simply that
you refuse to see them as anything other than monsters. That is, it sounds like you literally can't understand how someone can look at a Dragonborn or Tiefling and not immediately, intuitively, and without the tiniest shade of doubt think "that's a monster." If you truly can't understand that, you'll never be able to grok a group that wants those things. I find that kind of...disappointing, I guess. As I have said before, we have a game which lets us imagine any world we choose, and so of course we always imagine perfectly identical ones...
Orcs have a culture and a history. So do Hobgoblins (and in my game their history goes back way farther than anyone else's). Both are basically Human-ish variants, no more so than are D'born or Tieflings or Elves.
Doesn't make 'em PC-playable races.
Then what does? Seriously. How do you get into the club?
I can only refer to my Human Ranger's reaction in a 3e campaign when a half-Dragon PC came into the party: "I've spent most of my adult life learning how to kill things like this and now I'm expected to run with one and trust it with my life? You're kidding, right?"
Of course, it didn't help that our alignments were almost diametrically opposed...
Lanefan
Which is fine...
for that character.
But what about worlds where the next fishing village over was established by dragonborn refugees from some horrible war or other that never mattered to your apprentice-of-the-village-elder? Or ones where a peaceful merger of two kingdoms means that, technically, there are two royal families that always ritually marry each other--one human, one dragonborn--but must seek gigolos/concubines because they're not interfertile? Does literally
every character you make grow up "learning to kill these" sentient beings with a cultural penchant for honoring their deals? Does literally every world
ever consider them "things" and not people?
The antithetical alignment stuff certainly doesn't help. Especially if one of you was Good-aligned and the other Evil. Of course, this also means you actually gave them a chance to act, rather than immediately saying "MONSTER, KILL IT WITH
FIRE!!!" (Though, frankly, I cannot square being "Good" and being 100% okay with
instantly resorting to lethal force upon encountering a sentient of unknown disposition, regardless of how much one's been taught not to trust them.)
Heh. Yeah, if someone wants to play a tiefling but then complains about their character being presumed evil, that's when you start making... pointed critiques of their choices and tastes.
See, I don't even necessarily think this is correct either. Tieflings aren't assumed to be evil in Planescape, as I understand it--no more than Aasimar are assumed to be good. Because Planescape is intentionally a cosmopolitan world (in several meanings of the term!). I can certainly grant that "I physically
look like a demon, a creature known to be Pure, Living Evil," is going to mean that *many* settings will produce exactly this kind of knee-jerk distrust. At the same time, assuming that 100% of
all worlds that anyone could ever imagine WILL have that feature? How small the sandbox we choose to play in, when given all the beaches the mind might summon!