Comentario sobre Génesis 38:9

ושחת ארצה AND HE DESTROYED ONTO THE GROUND — He 'threshed inside and winnowed outside' (Genesis Rabbah 85:5).

כי לא לו יהיה הזרע, people in those days were extremely fussy about this point.

כי לא לו יהיה הזרע, he knew that not he alone would get the credit for performing this act of love for his deceased brother, seeing that part of the merit would go to his older brother seeing he had first fulfilled the commandment of marrying, so that he would share in the merit of having a son sired posthumously through a levirate union with his widow by his younger brother. [I fail to understand how someone who married in order to use sex only as an act of self-gratification can be described as having acquired a merit by the act of marrying. Perhaps gratifying the sex urge only with someone to whom one is married is by itself a מצוה, and this is what Seforno had in mind. Ed.]