John Calvin Commentary Genesis 38:13

"And it was told Tamar, saying, Behold, thy father-in-law goeth up to Timnah to shear his sheep." — Genesis 38:13 (ASV)

And it was told Tamar. Moses relates how Tamar avenged herself for the injury done to her. She did not at first perceive the fraud but discovered it after a long time. When Shelah had grown up, finding herself deceived, she turned her thoughts to revenge. And it is not to be doubted that she had long meditated on, and, as it were, hatched this design.

For the message about Judah’s departure was not brought to her accidentally; but because she was intent on her purpose, she had set spies who would bring her an account of all his actions. Now, although she formed a plan that was base and unworthy of a modest woman, this circumstance offers some alleviation of her crime: she did not desire a connection with Judah except while he was in a state of celibacy.

In the meantime, she is hurried by a blind error of mind into another crime, no less detestable than adultery. For adultery would have violated conjugal fidelity, but by this incestuous intercourse, the whole dignity of nature is subverted. This ought to be carefully observed: those who are injured should not hastily rush to unlawful remedies.

It was not lust that impelled Tamar to prostitute herself. She grieved, indeed, that she had been forbidden to marry, so that she would remain barren at home; but she had no other purpose than to reproach her father-in-law for the fraud by which he had deceived her. At the same time, we see that she committed an atrocious crime.

This often happens, even in good causes, when anyone indulges his carnal affections more than is right. What Moses alludes to regarding garments of widowhood pertains to the law of modesty. For elegant clothing that may attract the eyes of men does not suit widows. And therefore, Paul concedes more to wives than to widows, as wives have husbands whom they should wish to please.