The studies mention driving and the operation of heavy machinery specifically, but I'm pretty sure sex falls into that category too. One of the long term effects of heavy pot use on men is a decrease in levels of the hormone testosterone and lower sperm counts. This can lead to a lower sex drive and fertility problems. In women, one of the long term effects of heavy pot use is an increase in testosterone levels which can cause infertility. For both sexes the long term use of pot is associated with a diminished or extinguished sex drive.
Alcohol can also cause problems with coordination which can make having sex difficult.
The great irony of alcohol is that while it increases the desire to have sex, it decreases the ability to perform. Men under the influence of alcohol can have difficulty achieving and maintaining an erection. Recent studies in women have suggested that it lowers their arousal levels and their ability to achieve orgasm. Long term heavy alcohol use can damage the central nervous system and cause impotence. Plus, the big ass beer gut doesn't do much for you either.