parshanut torah comment - Where does the Talmud discuss the reason for Onan's death?

Genesis 38 says that instead of impregnating Tamar, Onan let his seed go to waste, and that this angered God, and therefore God killed Onan.

I have a vague memory of reading somewhere in the Talmud that there is a discussion to determine the exact reason for his death. The accepted reason was that his seed was wasted. Where exactly would this Gemara be? Or am I remembering something different?