Cancer treatment, especially chemotherapy, radiotherapy, and surgery, can affect your ability to have children (fertility) in different ways:

Orchidectomy
Having one testicle removed won’t affect your sex life and usually won’t stop you from having children. But if you need chemotherapy or if your other testicle isn’t making enough sperm, your doctor may suggest sperm banking before surgery, just in case.
Chemotherapy
Chemotherapy can lower your sperm count, which might make it harder to have children for a while or, in rare cases, forever. That’s why your doctor may suggest sperm banking before treatment.
There’s no evidence that chemotherapy affects future children, but small amounts of the medication could be in your semen during treatment. To protect your partner, it’s best to use a condom during treatment and for a short time afterward.
Radiotherapy
Radiotherapy helps lower the chance of seminoma coming back in the lymph nodes in your belly. Radiotherapy usually won’t affect your ability to have children, but your doctor may still recommend storing sperm just in case.
Retroperitoneal Lymph Node Dissection
Retroperitoneal Lymph Node Dissection (RPLND) is surgery to remove cancer from the lymph nodes in your belly. It can sometimes affect the nerves that control the release of sperm, causing it to go into your bladder instead of coming out of your penis (this is called retrograde ejaculation). If you haven't already, you may want to store sperm before surgery.