I really admire the look of an Afro -- and I agree with the first commenter that people are going to be drawn to touching it because it's soft and pretty. I am tempted to do the same.
But the fact is that I have no right to approach you and put my hands anywhere on your body without your permission. Women's bodies are not public property, and this includes their hair. You have a right to your own space, your own bodily autonomy, and that includes people messing with your hair. I suppose we should be thankful that the people are at least asking if they can touch your hair, instead of just bald-faced walking up to you and putting their greasy hands right in your Afro. A lot of women experience this phenomenon when they're pregnant: people walk up to them and just plop their hands on her belly without asking. It's rude and people need to stop thinking they have a right to invade your space to satisfy their own curiosity.