I finally found the etymology, and I am hard put to comprehend how such a stupid
misconception could have arisen!

sperm - c.1375, probably from O.Fr. esperme, from L.L.
sperma "seed, semen," from Gk. sperma "seed," from speirein
"to sow, scatter," from I.E. *sper- "to strew." Sperm whale
(1830) is a shortening of spermaceti whale (so called because
the waxy substance in its head was mistaken for sperm), from
spermaceti (1471), from M.L. sperma ceti "sperm of a whale"
(from L. cetus "large sea animal"). Spermatozoon is an 1836
formation from Gk. spermato-, combining form of sperma (gen.
spermatos) + zoion "animal."