John Calvin Commentary Genesis 38:10

"And the thing which he did was evil in the sight of Jehovah: and he slew him also." — Genesis 38:10 (ASV)

And the thing which he did displeased the LORD. The Jews speak less delicately about this matter. I will content myself with briefly mentioning this, as far as a sense of decency permits discussion. It is a horrible thing to pour out seed outside of intercourse between a man and a woman.

Deliberately avoiding intercourse, so that the seed falls on the ground, is doubly horrible. For this means that one extinguishes the hope of his family and kills the son who could be expected, before he is born. This wickedness is now condemned by the Spirit through Moses as severely as possible: that Onan, as it were, through a violent and untimely birth, tore away his brother's seed from the womb and, in a manner as cruel as it was shameful, has thrown it on the earth.

Moreover, in this way, as much as was in his power, he tried to destroy a part of the human race. When a woman in some way expels the seed from the womb through artificial means, this is rightly seen as an unforgivable crime. Onan was guilty of a similar crime by defiling the earth with his seed, so that Tamar would not receive a future inheritor.