How many people marry their first gf or bf?

So I've been with my first girlfriend for about a year and a half and I'm very serious in regards to our future. We are a very good couple, and I have no desire to "test the waters" and see what other women are like (why would I if I'm happy?). We never have serious fights, and we are still very sweet with each other.

I am her third serious boyfriend, and she is very happy in our relationship.

Obviously, with both of us very happy and in love, we are looking at the eventual possibility of marriage. But my main question is does this happen often? Do people often marry their first girlfriend or boyfriend happily?