As I posted earlier the chances are 2% for permanent impotence and 10% for temporary impotence. They are slight risks but risks nonetheless.
There have been posts in the Men's Health Forum by those rendered permanently impotent and/or with serious retrograde ejaculation issues. Those threads/posts are there if you want to search them.
As I mentioned earlier banking one's sperm solves the reproduction issue. The issue boils down to the risk of losing sexual function or having it impaired.