Many valid points have been mentioned here so far.
I also think it depends what Sith you play. Do you play a pureblood Sith? Then a lightside char is somewhat not overly realistic
, because it is in the nature of their race to be evil (imho).
However, I play a Twi'lek Sith Sorcerer. I chose it, because it thwarts the Sith way. Twi'leks are the lowest alien breed there is (in the eyes of a pureblood Sith) and their biggest worth would be serving as slaves. The storyline sometimes reacts to my char being a Twi'lek, however only slightly. In such a context, I find it absolutely unbelievable to play a dark side Twi'lek Sith. Or a Sith at all, but that's another topic.
So, all in all, find your own context in the game and then dark or light side will fit to it. And please don't give a cr*p about what others think about your playing style. Have you not noticed that there are many id*ots playing this game? Don't let them spoil your fun. Always ask yourself this question: Why does the other even care for it? Why are you an issue for him/her? If (s)he would get along with him/herself, (s)he would not bother whether you are ls or ds. Simple as that.