After I kidnap the beautiful princess, we will be married immediately in a quiet civil ceremony, not a lavish spectacle in three weeks' time during which the final phase of my plan will be carried out.
Aside from your lavish wedding having shit security (gotta look nice for the guests), details being public knowledge (giving the heroes time to form a plan since they know exactly where you will be at exactly a certain time), and just the hassle of having to go through wedding planning (such a chore), the civil ceremony will save you time and allow you to execute your plan at a time of your choosing.
This kind of stuff happens when your wedding is too well known