Interesting thread. To put my perspective on it - a hotwife / cuckoldress relationship with her lover / bull is like any other relationship. So its ending is just like any other relationship; there are many reasons, some are not that specific.
Being born with an undescended testicle (the main reason for having one testicle) only affects fertility and has a higher risk of cancer if it is left untreated (not surgically removed). Otherwise there is no difference in testosterone levels, fertility or penis size. If you have a small penis...