The poem
I saw a young snake glide Out of the mottled shade And hang, limp on a stone: A thin mouth, and a tongue Stayed, in the still air.
It turned; it drew away; Its shadow bent in half; It quickened and was gone
I felt my slow blood warm. I longed to be that thing. The pure, sensuous form.
And I may be, some time.

I believe that this poem symbolizes the male orgasm. Roethke's diction is definitely sexual and refers to the penis and male genetalia.
D00D,
shut up, not everything revolves around penises, you are type of creep that tries to find sexual imagery in everything, which is possible if you have that type of personality. What would be his purpose in writing about a penis?
just shut up and don't comment on respected poetry.
If this poem was about a penis, I do feel sorry for whoever it belongs to. I think he means that he wants to have the grace and stealthyness of a snake.