When a guy ejaculates or pre cums in his pants does that sperm or pre cum die once touching the fabric ? or is it still alive as they are not able to swim anywhere ?

As fish out of water: One can presume that a sperm without much semen is a bit out of his element. That means he won't function as well. In real life, people don't need to think about precum... because the whole reproductive thing is very simple. Sperm don't swim through clothes, but if one absolutely should not get pregnant, then use good birth control from start to finish... and never again worry about precum.