You can.
But that is not just an ordinary piece of paper. And ceremonies are
there to mark something important. They're an expression of the
couple's commitment to each other for life.
Here's an example of a marriage vow:
I, (Bride/Groom), take you (Groom/Bride),
to be my (wife/husband),
to have and to hold from this day forward,
for better or for worse, for richer, for poorer,
in sickness and in health,
to love and to cherish;
from this day forward until death do us part.
