There are a number of reasons, really, why this happens.
I will do my best to supply you with the ones I can... though there will always be more.
1.) Games are, despite all efforts to the contrary, still seen as being 'for kids'. Even though they've gone and had to create a system in which to rank these games, much as the motion picture industry did, the fact remains that the vast majority of the world views video games as being little more than overly glorified toys. And when kids get toys... the last thing their parents usually want them being exposed to... is six.
Which confuses me, as they will then proceed to leave the television on all day, which positively bombards a person with sixual suggestion... if not outright exposure to it. (depending on the show, this level can be anywhere from minimal, to outright pormographic.)
2.) six is always a touchy subject. I don't really know why... perhaps because we, as a species, are rather uncomfortable with ourselves. It's the rare nation, ethnicity, or group that can go the way of the Greeks or Romans and boldly ACCEPT six and sixuality as a part of daily life. Depending on where you go, these tendencies may be looser... or insanely strict. There's really no accounting for it, other than by basing your measurements upon the feelings of whoever is in power in a given area. In this case, morality and decency tend to be the words you will hear when debating this particular case.
3.) Feasibility versus 'taste'. Believe it or not, there is a certain amount of 'class' involved in the whole concept of six. Sometimes, game developers choose what is seen as a 'classier' way of handling the act... not because they're prudish... and not because they are forced to... but because it is the best way, given their medium, to portray the act. Other times, the tools they have to work with don't really LEND themselves well to graphic displays of sixual activity. Even if, as you say, they were not outrightly pormographic in nature.
Well... those are really the big three that come to mind, at the moment.
As I said, there are plenty more... however... those three are fairly good representations.
I'm sure other folks will provide (or already have, while I was typing this) more and better answers. But hell, maybe this will help.
Firstly, that's definately the most descriptive definition of the whole subject I've ever heard.
Second, I think they really just add it for sales, because in all the interviews for games such as Fallout, the question comes up "is there gonna be six this time???????", and when the director answeres "heck yeah!!!!! There's gonna be prosttutes!!!!", that just entices perverted 11 year olds to go out and spend $60 just to make it that far to the strip and pay just to get a blank screen. The sad part is, I'm kind've convinced that that's true.