Why is Sperm deemed as Najis while its the source that we and our children will born?

"[O, believers! Remember] When Allah covered you with a slumber To bestow you rest from His Presence And He caused rain to descend on You from the sky, to clean you thereby And to remove from you the impurity From Satan and to strengthen your Hearts and to keep your feet firm Therewith." Quran 8:11

It is narrated -regarding the above-mentioned verse- that what does Allah mean by the impurity from Satan is the semen.