Sperm & Marijuana – Science Magazine found results contrary to the commonly held belief that marijuana is universally bad for sperm production; rather, isolating compounds has somewhat surprising effects – “both hyper- and hypo-stimulation of CB2 disrupted the temporal dynamics of the spermatogenic cycle”. In other words, your cannabinoid receptors need to be adequately stimulated to produce optimal sperm numbers.

science – https://www.fasebj.org/content/30/4/1453
popular media write up – https://www.greenrushdaily.com/2016/04/09/marijuana-and-sperm/