One Last Fling

Like a bachelor party for fictional, not-yet-dating couples. The narrative often drives the Star-Crossed Lovers into a relationship or fling juuuuuust before they finally hook up for good. Perhaps so they can sow wild oats or learn important lessons about their partner, or on the opposite end of the scale, who to trust with an unattended drinks. Sometimes these last minute hookups are used to get the other party to realize the error of their delays. Often, this is the very first relationship in a long time for the flinging party, since they've been so entrapped by the ongoing will-they-or-won't-they nonsense.

Examples of One Last Fling include:

Literature

This kind of fruitless love is in the air in the second-last Harry Potter book, just in time for most of the relationships to be dashed and properly reordered in time for the finale.
Jacob and Bella in Twilight, just in time for Bella to decide that she really needs to be with Edward.
This is pretty much the relationship between Alanna and Liam in Lioness Rampant, since Alanna has decided to travel after breaking up with Prince Jonathan and she meets him on the road. They eventually realize that their goals are too different and split up amicably. Liam soon dies defending the palace at Jonathan's coronation, and Alanna ends up with longtime friend George Cooper in the end.

Comics

In the gay-themed, furry, Slice of Life comic, Circles, Paulie explains about his past in issue #5, Paulie said that one last fling couldn't hurt and had sex with Keith. However, this became the biggest mistake of his life as this caused him to contract HIV from Keith.