There's nothing like getting accused of racism to liven up one's day.
There are two separate arguments on this thread: One, whether there is some correlation between skin color and promiscuity, and two, whether there is too much promiscuity among the women portrayed in the comic.
The first is easy to answer: The only considerations that I give to skin color is that there be a normal distribution among a group, except in the case of a blood relation to an existing character.
In the case of Sabine, she is the tone that she is for two reasons: One, because it is a "mixed race" tone that could pass for any ethnic group in a pinch, as befits a shapeshifter, and two, because I wanted there to be at least one person of color on the Linear Guild. Yikyik, Thog, and Zz'dtri all needed to be the colors that they were due to their species, and Nale needed to be the same color as Elan. So, I could have made either Sabine or Hilgya dark skinned--either way, a black woman would have been having sex. I chose Sabine because Durkon is dark-skinned, and I wanted there to be some difference between Hilgya and Durkon to show that dwarves have races, too.
In the case of Jenny, I established the character as a throwaway joke in On the Origins of PCs, at which time I chose her to be black purely at random. (You'll note that Billy the Thieves' Guild Intern is also black.) While I expected the Thieves' Guild to come back as a whole, I didn't get the idea to have her sleep with Belkar until I was plotting this arc. At that point, I suppose I could have invented another female thief character out of some misguided "Character Affirmative Action" or something, but really, it didn't even cross my mind to do so. I had a female character with a high enough Charisma to be a bard/sorcerer (and thus attractive enough for Belkar to kiss her rather than kill her), why create another throwaway when I can use the one I have? Besides, going out of my way to NOT have a character behave inappropriately due to their race is just as racist as if I had intentionally made her black because she was going to put out.
And in the case of Sarah and Julia, they have dark skin because Roy has dark skin. Which is obvious, so it asks the question as to why they act the way they do. Well, Sarah is promiscuous in the afterlife because seeing your mom be promiscuous is uncomfortable and awkward, and I wanted to make that joke. Note that Sarah was pretty monogamous in life, however; she met her husband at age 19. Julia is not necessarily promiscuous, merely shallow and vain. She wants attention, and she uses her looks to get it. This is intended to contrast with Roy, and shadow Eugene's egotism.
So that brings us to the issue of whether or not the women of OOTS are all unnecessarily promiscuous, to which I would say a resounding "No, not in my book." (I mean, it obviously, IS my book, but you know what I'm saying.)
The characters in the comic, for the most part, are relatively young people (in their 20's, for most of them) who live a life of constant danger and travel. They do not have the luxury of going on 10 dates before hooking up, because they spend hardly any time in cities or towns at all and their prospective mate might get eaten by a wereplatypus before that special 10th date. Their profession is such that they may only get one chance at that special connection before fate rips it away from them.
Haley knew Elan as well as anyone by the time he grabbed her and kissed her. Given that she knew her own feelings well, why should she wait at that point? What if something crazy and unpredictable happened the next day like, say, Xykon invading the city and separating them for six months? Celia may have been a civilian, but she knew she had to return to the plane of Elemental Air the next day to resume her studies and that Roy may be killed before she saw him again. As it turned out, that is exactly what happened. So how can anyone fault either one of them for rushing in?
We all live in a fairly sedentary world, where long violence-free lives are the default and we can date someone for months or even years before having sex with them. The characters of OOTS do not have that luxury. Faulting these women (or any of the men, really) for knowing what they want and going after it strikes me as projecting morals that were built for this world onto theirs.
As far as the prevalence of sex in OOTS at all, well, that's intentional. It's a great source of jokes, and it's something that has hardly ever been addressed in other works based on D&D. A frank look at the likely ramifications that a D&D-style world would have on sexual behavior is one of the secondary themes of OOTS, if a stick figure satire comic can be said to have themes at all. This isn't the medieval world, after all, it's a world with 100% gender equality, a known afterlife, clerics that can cure any disease, and rampaging monsters around every corner.
If you're looking for a story where the main characters never talk or think about sex, much less have it, you should go and read...um...every other work of D&D fantasy fiction ever produced.