The study suffers seriously from response bias. Everybody responding feels like they have something to prove. You can speculate all day about whether people are guessing high or low, and its probably a little bit of both, but in the end the survey is totally meaningless for drawing any conclusions from.
23 sounds abnormally high to me as well, but this is coming from the guy with 1 partner ever and who has bee with his wife for over 10 years (since I was 14).