There are many ways to answer this question.

The simplest is that many statements of Chazal clearly show that Yehuda did not sin when he had his relationship with Tamar. This is clearly the gist of RASHI'S statement (Bereishis 38:26) that some opinions say Yehuda stayed married to Tamar, which he could not possibly have done had he thought she was forbidden to him. Additionally, the Bereishis Rabah (85:5) says that Yehudah was actually the first to start the Mitzvah of Yibum. Though the Mitzvah of Yibum in the Torah is only for one to marry his brother's widow if they did not have children, the YEFEI TOAR on the Midrash (ibid.) explains that before Matan Torah it was looked upon favorably to marry any deceased relative's wife who did not have children, and Yehuda started this process.

Even though Yehudah did not initially know who Tamar was when he had relations with her the first time, we know that great Tzadikim, before Matan Torah , could tell that if they would have relations with a certain woman l'Shem Shamayim positive things would result. Some say that this was the reason Yaakov married two sisters. Rashi even says that the wife of Potifar was "l'Shem Shamayim," as she saw through astrology that she would have great children if she were to be with Yosef (however, that relationship was forbidden even before Matan Torah). It is therefore reasonable to assume that Yehuda also had such intentions when he initially had relations with Tamar. This is supported by the Bereishis Rabah (85:8) which says that Hash-m sent the Malach in charge of desire to steer Yehuda towards Tamar, and the Malach said "Yehuda, where are you going? Where will the kings come from, where will the great ones come from?" This was notably not done by the Yetzer Hara, but by a special Malach sent from Hash-m. Though this came about in a strange way, we should realize that Yehuda's actions are not considered a sin.

All the Best,

Yaakov Montrose