Does sperm die when in the lining of underwear?

Sperm cycle of life: The spermatozoid need an specific environment to survive, this enviroment is inside the vagina. Without ejaculation of semen into the vagina, conception is very unlikely. Not zero risk, but the chance of pregnancy from any single event like this is very low. If the ejaculation is on the underware the spermatozoids are not viable.

Yes. : Sperm live outside the body for different lengths of time, depending on the environment in which they were released. Unless they are deposited into a female reproductive tract, sperm cells are easily damaged and can only survive a few seconds to a few minutes outside of the body.