Say what now? I'm not going off topic, Bioware made "Jade Empire" before they made "KOTOR". It's the gateway that may lead to KOTOR being released. I've actually never played Jade Empire but the reviews seem good.

KOTOR 2003.
Jade Empire 2005. Special Edition 2007.

.... Really? Huh. One moment.

^QFTA


Back on topic : I think sith are ugly because people-consummer are so stupid that it must be very clear who the villains are in the [manichean] starwars stories ....and let's face it ...ugly people are the evil incarnate, right ?


Sidenote : there is sarcasm somewheres.

Back on Topic:

In Star Wars the Old Republic MMO the Master of the playable Sith Inquisitor is a rather beautiful (De gustibus non est disputandum) blonde female Sith looky looky here
Seeing her, I immediately thought that she might be a possible BioWare Standard Romance option but I think this is not the case here.

This does prove my point somewhat, that most Sith chose deliberately to look sithty (Typo or not - whatever)

As far as I know there is no official stance on this topic.
But there are two people called the 'Sith', namely the original red skinned aliens with a penchant for the 'dark side' of the Force and the self proclaimed 'Sith' that are really just dark Jedi with a certain set of rules (one of which is silly/cool names starting with the Darth appellation).
Now the 'new' Sith use bits and pieces of the old Sith culture and dark side techniques, as well as the name obviously.
To me the dark side corruption is obviously meant as a plot device or an easy way to tell who's good and who's bad and as such there are no specific 'rules' for how and when it happens.
Now perhaps all the sexy Sith are using Force illusions to keep up their appearance or perhaps, and this is something I was thinking of, perhaps it has to do with the mental state of the dark side user.
So Darth Tyranus for example is still normal looking because although he uses the dark side as a means to an end, he is not actually 'evil' to his core. Whilst Anakin being extremely self-centered and full of anger and hatred starts changing quite early (even before being crispy).

What does the Quantum physics Field Theory Award has to do with the aforementioned quote?

What is this Force illusion? Just another form of Star-Warseque Make-Up. Something like plastic-surgery done by the Mediclorians not by some overpaid/cheap physician.
Again the rest of your pretty valid point just show that Ugliness depends on mental state or rather stance which is deliberately chosen.
So it all boils down to this:
They want to look ugly, they chose to look ugly and most of the time they probably enjoy it. A lot of Sith are proudly bearing their scars of recent battles, just like any honorable Mandalorian would do.

If you're saying my points boil down to that conclusion, that would be incorrect.
If however you're saying you feel it boils down to that, well that would be your opinion.

I don't want to indulge in any pettifoggery here,
what I am saying is that actually any Sith caring for his or her outer apperances could get pretty or at least less ugly if they chose so. Maybe even Malak could lose his resemblance to Zbigniew Krycsiwiki (Jaws the cool James Bond Villain) but he is to powerhungry to even remotely care.
Sometimes this isn't happening, as in their current mental state, some of the Darth's out there are simply batshit crazy, they maybe even use revolting appearance to shock and question general established views. Like those Emos are trying very hard with their stupid hairstyles.

I would argue that any Sith who wants to look pretty actually can look pretty.
Saying it was intended by George Lucas because Sith = Evil = Bad = Ugly is an unfair and far to easy way of an excuse. I don't accept that as an answer.

So they either look ugly because the force changed them (then why do some look pretty?) or because the chose to do so.
The reason why they chose to look ugly are probably interesting and might be worth of further discussion but the answers to TinyE's questions is straightforward:

Because they want to be ugly and disfigured