When love is described as a fist, it seems like it has physical power, and that power is a masculine one. It is a man's fist, after all. It appears this dude is totally in control.
With hard knuckles (3)
This is no weak fist. It has hard knuckles and has probably seen a fight or two before. The question is, do we think this fist is real? It has been described as a metaphor for love, but the more real qualities it gets, the more real it seems.
Crushing the lips, Blackening the eyes (4-5)
These lines give us a clear demonstration of the exact physical effects that the power of love, and a man's fist, can have on a woman. It's the first moment in the poem where a physical power struggle becomes apparent. We can call this a metaphor all we want, but it's hard to deny that someone is getting punched here.
Hit me again, Says Clorinda. (6-7)
While all of the power has so far been in the man's hands, here, we see that the woman has something to say about it after all. What she means isn't clear, but she has taken some control by speaking. Though she has suffered a punch to the mouth, she still manages to make her words heard. But is this a good thing? Or will it just cause her more injury?