It is impossible to forget your first love. Even if it is not particularly memorable, a first love is a formative experience. No one will ever forget something that is so new and so important in their development. Usually it is a good experience, and no one wants to forget it.
I do not think anyone actually forgets their first love, they move on and find better people or a new love, but they still look back and smile on their first. It does not mean they are stuck on the person, or want it back it is just a sweet teenage memory to keep.