Genesis 38:15 - Ellicott's Commentary On The Whole Bible

Because she had covered her face. — The Jewish commentators all agree that this was not the custom of harlots; and as Judah, in Genesis 38:21, calls her kedeshah, one consecrated, he probably thought that she was a woman performing the vow required of every female votary of the Phœnician Venus (Astarte), once in her lifetime (Herod. i. 199). Hence the hire was a kid to be sacrificed to the goddess. As for Tamar her object was to assert her claim to the inheritance of ‘Er. Lange considers that the wickedness of ‘Er had caused him, equally with Onan, to neglect her, and that consequently there was no real incest. This is made probable by her immediate conception.