When a woman commits fornication and then weds her lover, does the sin of fornication remain or is it removed simply because they marry?

Fornication is one of the major sins1 that is removed by repentance. It is not a condition for repentance that the fornicator marries his lover. Rather, repentance is by desisting from such an act, regretting committing it, and resolving never to return to it again. God forgives whoever repents whether or not he marries his lover; repentance is not bound to marriage even if common decency demands protecting the woman with whom he has erred2. If both of them repent and are suitable for each other, then it is preferable that they marry each other.

And God Almighty knows best.

Footnotes
1. Sins are usually divided into major and minor sins. The minor sins are removed with certain virtuous acts such as making wudu' and praying. Major sins are only removed, in the opinion of most scholars, by sincere repentance. Some scholars hold that an accepted hajj also removes major sins.
2. By preventing others from discovering she is not a virgin.