"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."