Yichusdik: As a general rule, before Matan Torah, when the story of Yehuda and Tamar took place, we were not bound by the Torah. It was kept on a voluntary basis. (rofeh hametzaveh el hacholim) When one saw a spiritual opportunity, one could take advantage, even if it was not within the letter or spirit of the law.
After Matan Torah, we have no such license, unless with a navi, and a hora’as sha’ah, according to the principals of what Eliyahu did on Har Hakarmel. (Melech hametzaveh al ha’am)