I finally got some time to play KOTOR, but not enough to replay it multiple times. While the most differentiating choice is between the light side of the force and the dark side, the choice of gender is probably also important (class as well, but I think I know which ones I choose there). So if I want to experience as much variety in the game as possible by playing only once for the light side and once for the dark side, which gender makes most sense for which side, and why?
So, the short answer is that you want to play things through once as each gender absolutely, since there is a romance option for both.
As to the relationship between each gender and your choices with regards to alignment... Well, going through with each combination Light/Male, Dark/Male, Light/Female, Light/Male is going to have a unique take on certain events at the endgame.
That said, the difference aren't so substantial that it won't be fairly obvious what those differences are once you get there.
I will say that if you enjoy playing up the somewhat cartoonish tone of evil of KOTOR's dark side path, play as a Dark Female. More detailed spoilers follow.
Basically, there are two romance options. Female characters can fall for Carth, who is staunchly Light Side aligned through the end of the game. If you play as a Dark Side female, you get to completely torture and abandon Carth as he spends the entire game trying to save you. Male characters can romance Bastila, who is turned to the Dark Side later by Malak. In the end, Light Side players can choose to redeem Bastila, or to kill her. Dark side players can choose to make her their apprentice, or, again, to kill her.
To put it another way:
If you prefer that your Light Side playthrough emphasize redemption, play male. If you prefer that it emphasize loyalty, play female.
If you prefer that your Dark Side playthrough emphasize cruelty, play female. If you prefer that it emphasize power and domination, play male.