Folks, we're not saying that the snake = sex in this poem. Then again, we're not saying that it doesn't equal sex, either. You know who else isn't saying? That's right: ol' long-dead Emily Dickinson. So, that leaves scholars and critics to speculate about Dickinson’s sexuality and romantic experiences through her poetry. Unfortunately (or fortunately, depending on your attitude), there are so many poems and letters that present conflicting interpretations that it is really up to you to decide. With “A narrow Fellow in the Grass” specifically, it's hard to tell how far to push implications of sexual content. Sure, if you look for sex in the poem you can find it. Sure, the snake could easily be seen as a phallic symbol, but then again it could just be a snake. Who's to say? Well, you are, ultimately. Still, if you are leaning in that direction, we can assure you that the element of sex can be found here. You just have to look for it.
Sex, to the speaker, is a great unknown. He can only guess at it through this elaborate snake-encounter story, which is why it's so scary. Boo!
This poem is not about sex per se, but rather about personal intimacy. The speaker is describing his difficulty in maintaining a friendship with a particular person (the "Fellow" in the last stanza).