According to Rashi:
ותשחת [THE EARTH] WAS CORRUPT — It means lewdness and idolatry, as (Deuteronomy 4:16) פן תשחיתון “lest ye deal corruptly” (the following words show that this refers to idolatry) and as ‘כי השחית כל בשר וגו “for all flesh had corrupted etc.” (in next verse of this chapter where Rashi states that this has reference to lewdness) (Sanhedrin 57a).