Our speaker is really enthusiastic about flowers. Or at least this particular flower. You might be able to picture him, sitting at the edge of the garden mentioned in the poem, just enraptured by the fact of the poppy and all that it represents, by the bee, and by the fly. In fact, these things are so incredible to our speaker that he goes on to extrapolate (to infer, or to extend) what he sees to all kinds of interesting things about the life cycle, sex, and femininity, just by observing this one seemingly innocuous (innocent, easily overlooked) plant. He's so enthusiastic that it's catching – we keep using three-point SAT words in this description here.
In fact, our speaker is so taken with this flower that it almost seems to cause him to walk a fine line between joy and despair – he focuses quite a bit on the imminent death of the flower itself (see lines 16-20), and even goes so far as to make up a bit of an after-death speech for it. Thus all the praise, while earnest and intense, is also fragile in a way – the speaker knows what is about to happen to the thing that he's so awestruck by, and it's almost too much to bear.