gleams in all its power. Otherwise the curved breast could not dazzle you so, nor could
We get more enjambment as the line from the previous stanza flows into this one. The speaker now embarks upon a rather confusing list of attributes (grammatically speaking, at least).
Remember, he just told us in stanza 1 that the statue gaze is not absent, it's just turned "to low." That's how powerful this statue is—even without a head, you still get a feeling of the intensity of Apollo's gaze, which "gleams in all its power."
As well, the speaker told us how the torso itself "is still suffused with brilliance from inside / like a lamp" (3-4). It's this inner light that allows all the following things to happen (beginning with "Otherwise" in line 5).
Because the statue has this mysterious light of its own, the speaker—who addresses us as "you," as though we're looking at the statue with him—is dazzled by the torso's elegant form.
a smile run through the placid hips and thighs to that dark center where procreation flared.
That light is so powerful that the statue's vibrant sense of life also creates a "smile" in the peaceful ("placid") lines that lead to the statue's, er… nether regions.
There's something playfully sexy in this figurative description, but there's something also deep and serious here, in the way in which the speaker refers obliquely to "that dark center where procreation flared." He tactfully doesn't describe the statue's private parts, but instead, merely suggests the mysterious ("dark center"), yet powerful ("procreation flared") nature of sexuality.