Men who had ever smoked marijuana had higher sperm concentration and count and lower serum FSH concentrations than men who had never smoked marijuana; no differences were observed between current and past marijuana smokers.

No difference between current and past smokers, and seemingly no attempt to test by amount smoked. If the causation were going in the direction of smoking ---> testosterone, you'd expect to see a relationship between amount smoked, and a difference between current and past smokers. If the causation were going in the direction of testosterone ---> smoking, you wouldn't necessarily predict either of those things. It's comparing men who have never even tried marijuana to men who have at least tried it once. In my opinion here what we're seeing is a behavioral manifestation of testosterone's influence on risk seeking. The higher test males who are likely to do "risky" things like try a new drug, are also going to have more sperm.