פירוש על עבודה זרה 72:7

In Genesis 38 Judah suspects that Tamar has had sex outside the framework of marriage. The simple reading of this chapter is that Judah is accusing her of quasi-adultery, but the rabbis read it as non-marital sex. The fact that he states that she is to be penalized for this, implies that sex with a non-Jew is prohibited (he assumes that she had sex with a non-Jew because there were not really that many Israelites around then). So how can we say that the students of Shammai and Hillel initiated this prohibition?