Vrangalova is the first to admit that the Casual Sex Project is not what you would call an objective, scientific approach to data collection. There is no random assignment, no controls, no experimental conditions; the data is not representative of the general population. The participants are self-selecting, which inevitably colors the results: if you’re taking the time to write, you are more likely to write about positive experiences.
So, its an art project, then? This is also why self-selected polling is never used as anything more than a starting point. The people who participate in stuff like this will inevitably fall on the "OMG AWESOME' side or the "OMG EVIL" site with very minimal activity in the center where most people live.