http://edition.cnn.com/2016/10/30/health/male-birth-control/
I mean, one would think that anyone with the minimum sense of logical thinking would understand that it is way easier to prevent 1 (as one, singular) single egg to get fixated in the uterus during the course of 28 days, than to prevent millions (as in > 1000000) of spermatozoids to be produced every single day.
But perhaps it's just me that actually looked at the obvious numbers...