Those results should be taken with a grain of salt. They are based on interviews which cannot be verified. People can say what they want, especially in such a delicate case. Nobody checks if the interviewee really says the truth. People are not laboratory mice that are all the time under surveillance.
And the alleged correlation between the frequency of sex and the development of wages could be pure coincidence. Those "post hoc ergo propter hoc" studies (Latin for "after this, therefore because of this" wikipedia ) are therefore controversial.
But the results are plausible anyway. People who have more sex are usually less frustrated. They are more balanced and have less incentive to burn spare energy through arguing with colleagues and superiors. They even might smile more often. Therefore people with a high frequency of sex could be more cooperative and more popular, attributes which could lead to wage gains.
MarketWatch also quotes a psychiatrist, who says that people with higher self-esteem and self-confidence, could attract more sexual partners and more work opportunities. In this case a higher sexual activity is not the cause of better wages, rather wages and sexual frequency are both results of a third factor, the self-esteem and self-confidence.
Anyway, I recommend to test this thesis and to have more sex. Even if that doesn´t result in a wage raise it certainly leads to more fun. Post hoc propter hoc.