ཧོ་འོ་ ཐག་~རིང་གི་ ཨ་~ཇོ ། རེ་~སྒུག་~གི་ ལོ་~ཟླ།
The purpose of his having intercourse with his brother's wife was to provide a child for his brother, and Onan explicitly disobeyed that requirement.
Family, Obligation, Conflict