The poppy in "Big Poppy" is very clearly a woman. Since the poppy is so intensely personified throughout the piece, there's a lot going on here that deals with the idea of women and sexuality, women and love, women and their appearances and behavior…tons of things. The poppy is given various body parts and clothing, and the way in which they interact and the way in which the speaker reacts to his own creation of these symbols make femininity one of the overarching themes of this piece.
The femininity of the poppy in "Big Poppy" seems to posit women as blithely unaware of, to the point of being passive about, their own beauty.
"Big Poppy" plays with the woman-as-flower cliché by inverting some of the gentle stereotypes that go along with that kind of image; instead, the flower is hot, fierce, and wild.