A Star Wars encyclopedia and some flashbacks in the Jedi Apprentice book series recount how Qui-Gon Jinn and Tahl first met as children. Tahl started training alongside Qui-Gonn at the academy located in the Jedi Temple on Coruscant when she was 6 years old. The friendship they quickly developed eventually culminated in a duel that attracted an audience of Jedi Masters interested in its outcome.
From this point on, Qui-Gon and Tahl are closely linked. After they both attained the rank of Jedi Master, they would undertake missions for the Jedi High Council together with some regularity. Since Jedi are explicitly encouraged to reject their own feelings for a possible love interest, it’s unclear precisely when Qui-Gon and Tahl developed their mutual interest in one another. Based on the fact that their bond dates back to childhood, however, it’s reasonable to assume that their shared love for each other that they confess in “Jedi Apprentice: The Ties That Bind” was suppressed for decades before finally seeing the light of day.