when someone asks you that, just laugh and say "Why would I want to tie myself down when I am having so much FUN!!!"
I agree totally with pocoellie. When the time is right, you will know it.
Just wait until you do get married, then you will get the nosy questions about "When are you going to have a baby?"
Then when you do have one, "When are you going to have another one?"
Let's face it, there is no end of nosy people in the world.