"Are you sure you're ready?"
We hear this question pretty frequently, and really, it's often a projection of the questioner's own fears about commitment and lifelong relationships.
Take the high road: "We've been talking about getting married for several months now and realize it's the right step for us. We're both ready and pretty excited about it."
Go ahead, be snarky: "Well, not entirely. But I really want to try on the poufy white dresses."