sure why not. i know about five couples for whom they are each other's 1st. all of them seem to be great stable relationships. one is my wife's parents. they've been together for about 40 years. another is a couple who are in their later 40s and seem strong as ever. the other 3 are early to mid 30s and again are all going strong
Obviously, there are many examples of people marrying their childhood sweetheart and living happily ever after.
However. . . I think that some people want to make that happen - like they have a fairy talel notion of what their life should be - and they try to force that outcome with someone who is not right for them. After 8 or 10 years, they finally realize that this match really was NOT meant to be.