The it refers to the instrument and the person.

Notice how the speaker repeats the word longer between two commas, slowing down the rhythm to savor the moment and indeed make you take longer reading.

The hunger simile works since the arsehole is the mouthpiece of the instrument.

The notes are presumably the sounds that the addressee makes as the speaker plays him or her.

The alliteration slows you down when reading to imitate the pleas to last longer.

This is much more tender than the description of a tightening throb when fucked.

Now that the speaker is addressing the person rather than describing an arsehole, he gets much more loving as opposed to the octave’s primal sexuality.

This line marks a shift, which is typical of Petrarchan Sonnets.

The octave described the arsehole interestingly, and now the speaker will address the person directly, speaking as I.

This is quite tender compared to the somewhat nastier descriptions of the last two lines.

This continues the violent language of the first stanza.

We’ve gone from comparing it to a flower and describing its color as violet, to comparing it to a bruise.

Albumen is another word for ‘egg white:’

Alternatively, albumin is the most common protein in human blood.

This could be a reference to Palgrave’s Golden Treasury, an anthology of English poems.

The purse metaphor furthers the view of the subject as an article of clothing.

Further, describing it as a purse shows that the speaker values what’s inside. In other words, he wants to fuck it.

Here is the clothing comparison again, which we saw in the first line.

It’s a pretty clever description: Just imagine the person’s skin as a fabric.

This clothing imagery implies that the person was created as perfectly as a dress or an outfit— precise and perfect.

This is a pretty stark contrast to the dilating iris in just the last line.
This, along with the tightening throb description display the relative violence of the act.

This is a budding iris.