How does Romeo and Juliet's marriage in Act 2, Scene 6 foreshadow future events?

The fact that the couple secretly marries without benefit of Juliet's father's permission hints to us that something will go wrong because the rules for marriage have been broken. Paris follows these rules, but Romeo breaks them. The Friar's agreement to marry them is a huge faux pas on his part as well. 

The impetuousity on the part of the Friar, Romeo, and Juliet to marry foreshadows the later rash decisions of Romeo and Juliet to immediately kill themselves without thinking it through. Even the Friar's rashness in marrying the couple foreshadows his act later on of making the potion for Juliet to get her out of the marriage to Paris.

Of course, the fact that Romeo and Juliet are the only children of feuding families foreshadows that this cannot end well. Tybalt became very angry with Romeo at the party, and we know that his hot temper won't leave this alone. The marriage further complicates this situation, foreshadowing the duel to come later between Tybalt and Romeo.