There's also the risk that the drug is not as efficient as expected, and you end up with an unwanted pregnancy, so participating in this trial is never a no-brainer.
I looked up the study, and it seems that participants were not allowed to use a second method of birth control:
"Couples enrolled in the efficacy phase were asked to rely only on these injections for contraception."
And there were four pregnancies during the study (from the abstract of the study):
During the efficacy phase of up to 56 weeks, 4 pregnancies occurred
among the partners of the 266 male participants, with the rate of 1.57 per 100 continuing users
(95% CI, 0.59 – 4.14)
I don't think they were measuring the efficacy on the basis of whether or not the participants conceived - that measure is affected by too many variables. Sperm count is the more reliable metric with reasonable granularity for measuring the effect.