even if eliot were being subtle (which I don't think he is.  erudite, yes, subtle no)
he would use Paolo and Francesca. 
Montefaltro has acted upon something because he was given prior permission for it.  That act was sinful and even though he was given prior absolution, we find out in Dante that this absolution was worthless.  Eliot uses the GdM quote to show his speaker terrified of action -- for example: even if he "gets" all the hints, the woman may still throw off her shawl and say this is not what I meant, not at all.  Prior knowledge of an action's supposed results and consequences does not translate into real results and consequences.  Eliot was desparately and disparately trying to convey that in Prufrock (and indeed, throughout most of _Prufrock and Other Observations_ and _Poems 1920_.)