If I remove the head from my legal bull elk or buck deer, but I leave the antlers attached, does this meet "evidence of sex" requirements in Washington?

No. It is illegal to possess or transport big game animals unless evidence of the animal’s gender is left naturally attached to the carcass until the carcass is processed or stored for consumption.
For a bull elk and buck deer, evidence of gender includes EITHER: the head with antlers intact and naturally attached to at least one quarter or the largest portion of meat; OR the penis or testes naturally attached to at least one quarter or to the largest portion of meat.
Bull elk or buck deer taken in antler-restriction areas must have the head or skull plate, with both antlers naturally attached, accompany the carcass while in transit or in possession.