I would think it has got more to do with how satisfied they are with their jobs. If they are happy with their jobs, they'll want to have more sex. Rather than the other way around.
Exactly.
Or people who make more money are considered more attractive to their partners and have more opportunities for sex.
I can see why it helps. Sex generally puts people in a good mood, is known to stave off depression and anxiety (natural high/mood booster). People who are happy usually perform better in the workplace and tend to weather the stresses more adeptly. It only stands to reason that those are the people who would then be singled out for promotions, advancement and compensation/rewards. It's not the sex itself that helps you earn more money, but the boost it gives to overall satisfaction in life.