There's not a lot of sex in Paradise Lost, but Adam and Eve do have a famous love-making scene in Book 4. After the Fall, they have a slightly steamier sex scene that is more lustful and less lovey-dovey than its counterpart in Book 4. We also learn about Satan's son Death, who is born after Satan has sex with his daughter sin. Plenty of sex to go around here.
There is nothing incongruous about Adam and Eve making love; as Milton makes perfectly clear, love and sex are the same thing in the pre-fallen state.
Adam and Eve's post-fallen sexual encounter has affinities with Satan's encounter with his daughter; while there is nothing incestuous about post-fallen sex, it is motivated by lust rather than love.