Paradise Lost is about Adam and Eve's fall, the original sin! So it's no surprise that sin is a prominent theme in the poem. Don't forget that we also learn a lot about Satan's major sin (he tried to overthrow God) and a lot about the sins that Adam's descendants will commit. Oh, and Milton constantly reminds us of our sins, or rather of the fact that we are sinners in a fallen world.
Adam and Eve's sin is especially heinous because they were notified on numerous occasions (by God and by Raphael) about the rules. Such a blatant disregard for a simple rule almost seems worse.
Milton attempts to write a poem about a time when there was no sin; however, the poem is repeatedly dogged by discussions of sin, sinful responses to characters like Satan.