When Doom/Wolfenstein3D originally came out, the markets were quickly flooded with an array of knockoff clones. Some were decent, most were not. Ditto for Warcraft/Command&Conquer. So when the first half dozen or so games with similar "boy-man lives with hot mom and sisters, goes to school with hot teachers and coeds, much naughtiness ensues" themes started making bucketloads of money, of course there were going to be imitators,good, bad and indifferent.
In theory there's an infinite number of premises upon which to base an adult game. In theory, anyway. In practice, as people have mentioned in other threads, there are a finite number of assets/props such as buildings/interiors, furniture, etc, when it comes to 3d models. So in effect sci fi and fantasy games are somewhat harder to make, and more mundane contemporary urban/suburban settings are easier to make. So it's a boring suburban household--how do you make that hot? 1st, make the characters really attractive. Second, introduce a taboo element, like incest. 3rd, bring in other fetishes, so now what's "exotic" about the setting is what's happening beneath the surface.
Now, you say, there's already 150 games like that out there...how do you make one that stands out and still makes money? Great artwork and hot scenes. Great writing helps to amp up those hot scenes. And one other thing...put something new or different in there, a twist or turn. A little bit of voice acting, better than average animations, a sprinkling of humor, bonus content...any number of things can make a game really stand out.
