For those who are complaining that the game evolution is too slow, you have to consider that they are alone on a very small island. This is a very limited environment and it needs a slow evolution. If they started to fuck like rabbits on the first day, the story would be already over and it would be boring. I like also the idea of the experiment with liquids which influence the character. It reminds me of the 'Lost' show. It's original.
The only other option in this environment is to introduce new characters. For instance, they could have a visit from a wild tribe who kidnaps them and takes them to their island. The tribe uses them as sexual toys. It seems to me a very erotic situation. I didn't see any game using this topic. A long time ago i read the french novel 'Cruelle Zélande' from Jacques Serguine. It was one of the most erotic novel i've ever read. It's the story in the Victorian period of a prude english woman who is kidnapped by a wild tribe. She's trained as a sexual slave and of course little by little she becomes a slut.