Is it realism? The animation? The colors perhaps?
What make virtual sex games “good?”
Well, it depends on the goal of the game producer in some regards at least.
If true realism it to be achieved then photorealism is an obvious approach to creating a lifelike character in a virtual sex game.
Many virtual sex games are built with CGI graphics to meet these photo-realistic sex image requirements.
However, that realism is a cartoon character than the a realistic moving cartoon vixen is the answer. Animation becomes a strong factor in art contributing to virtual sex game development.
Sometimes a cartoon image can be very erotic. The realism is stripped away and just the eroticism remains. Several talented erotic artists have tried their hand at implementing their erotic art into a game. There are a few obstacle, of course…
For starters most artists aren’t software/web developers so essential skills that a game developer may need like coding and, at the very least, familiarity with game development platforms (Flash, Java, etc.) something many erotic artists do not have.
A variety of styles go into virtual sex games and because of that a variety of sub genres are created from the artwork. Some of these virtual sex games are very mature and some are more light-hearted and humorous. The style reflects that a lot.
CGI graphics tends to be used with the most mature games while 2D graphics that use light “cartoony” colors tend to go with the funny ones.
One thing is certain, a truly good virtual sex game depends a great deal on the art being attributed to it.