Removing one testicle does not necessarily impair fertility or sexual function. The remaining testicle can produce many millions of sperm and sufficient amounts of testosterone, the male sex hormone, which ensures that fertility is maintained. Since you mention chemotherapy I assume that you are referring to a case of testicular cancer. In that case it is worth mentioning that in some cases it is necessary to remove some lymph nodes while removing the testicle in which case the nerve pathways controlling erection and ejaculation can be disrupted. However, modern surgery is much more precise and less radical than the surgery that was performed even up to recent years. Therefore erection and ejaculation are preserved in the majority of cases of testicular cancer. Sometimes a man may have to have a testicle removed as a result of trauma or even torsion or twisting of the testicle and there are numerous case reports of fertility being totally unaffected in such cases. In a nutshell one testicle is sufficient to maintain fertility.