Removal of one testicle In men with testicular cancer, usually only one testicle is removed. This will not cause infertility and does not usually affect your sexual performance. Initially, after the operation, sexual positions which apply pressure to this area should be avoided. Some men describe orgasm as feeling different, and the normal contractions of the testicular sack (scrotum) at orgasm may make this feel uncomfortable. The amount of ejaculated fluid produced is usually less than before.
Removal of both testicles If both testicles are removed, for example, as a treatment for prostate cancer, the man will be infertile and usually unable to have an erection.
http://www.cancerbackup.org.uk/Resourcessupport/Relationshipscommunication/Sexuality/Surgery