I think the big problem is that combining cybersex and a game is extremely difficult.
Sex/romance is typically a creative process and hopefully collaborative
Gaming is almost always a goal oriented....challenge/reward system.
Their extremely different worlds.
I have heard mention scoring the other person. I like the idea but how many would continue to play if the were scored poorly.
Another idea would be to have people get a certain amount of money/influence points whatever that they could give away. You can't spend your own points on things but you can spend other players. Then you would have to create things that could be purchased with those points.
Its possible just seems that the whole idea really lacks a gaming feel to it. Otherwise its just a glorified chat program.
By the way from what I hear the Online Sims has turned into this already.
Second, if you are going for 'role playing' and sex, then you will need that rating system to guide each other on your actions. If you are hard to get, then you conversations will need to be that way. Stats are a key in role playing because it's a way to gauge the fake character versus the real, and how they are playing. Esentially, not role playing in a role playing environment is 'cheating'.
The problem is in that many people can't get into this mode, and a possible sex simulation would attract those who don't care to role play. If you don't care about role playing, then I guess they can play themselves, which again leads to the rating system.
Also, if sex is a major part, you need to consider stats and skills increase as actions are performed. Trying to smooth talk and get together with a high level player should be more worth while than a 'n00b' player who just started. Maybe even an experience system based upon performance.