Sure, this poem is a metaphor for love, but it's rife with physical actions and the body parts that perpetrate and are affected by those actions. The body imagery helps lend a sense of the very real threat of violence. When we're confronted with the fist, its knuckles, and the lips and eyes that fist bruises, we can't help thinking that the abuse in the poem is anything but metaphorical.
Line 2: Ah, the fist. What an image. We just can't escape it, and neither can Clorinda. This is the first we see of it, but it will have a lot to do for the rest of the poem.
Line 3: That the fist has hard knuckles tells us it's a rough and tumble fist for a rough and tumble guy. Maybe, just maybe, this man has seen a fight or two in his day.
Line 4: Now that fist is crushing someone's lips, presumably preventing them from talking at all, for the time being. One body part is causing serious damage to another.
Line 5: Finally, the fist blackens Clorinda's eyes. This fist has attacked both Clorinda's speech and sight by clocking her in the mouth and eyes.