For sperm to develop normally, the testicles need to remain at a constant temperature that is several degrees lower than elsewhere in the body. This is probably one reason why the testicles hang outside of the body. This is in contrast to womens ovaries, which are an equivalent reproductive organ. This need for a constant temperature is also why a mans scrotum contracts (shrinks) when exposed to very cold water.
For many years, scientists have known that exposure to high heat as found in saunas or hot tubs may lower sperm counts. But it should not block sperm production completely.
From your description, it sounds like you have azoospermia. This means that no sperm are found in your semen. Its highly unlikely that hot baths alone would cause this condition. Rather, azoospermia usually happens due to one of the following reasons:
- The testicles did not develop normally
- The testicles have been damaged, for example by certain infections (mumps) or exposure to chemotherapy or radiation
- There is a blockage in the sperm transport tubules
Most men who take hot baths or saunas will not have problems with fertility. However, if a man and his partner are having difficulty conceiving a child, it may be wise to avoid situations where the testicles are exposed to high heat.