By applying pressure on the perineum just before ejaculation, a man can induce retrograde ejaculation in which the semen is redirected to the bladder. This is obviously different from unintentional retrograde ejaculation, which is caused by some pathology.
I have read confusing things about whether inducing retrograde ejaculation is safe. Wikipedia for example says,
Many doctors also do not recommend [retrograde ejaculation] due to the risk of putting pressure on the pudendal nerve, which can cause numbness in the penis. 
Other sources I have found said an induced retrograde ejaculation could damage the sphincter or the prostate, or that doing it frequently over a longer period of time could cause a retrograde ejaculation to occur even when not induced, thus causing infertility.
No actual citation was given to any of this.
Is there any medical evidence about the safety of inducing a retrograde ejaculation? Does it cause short-term damage? What about long-term?