Or HaChaim on Genesis 9:7/אור החיים על בראשית ט׳:ז׳

1 פרו ורבו. Be fruitful and multiply! It was necessary to repeat this because when the Torah permitted resumption of marital relations when Noach and his wife left the ark (8,16), He gave permission to multiply. In this instance G'd commanded man to be fruitful and to multiply. Why did the Torah have to repeat this formula both in 9,1 and here in 9,7? Presumably in 9,1 the Torah introduced this commandment as a desirable action on man's part, a מצוה, whereas here it is repeated as a חובה, an absolute duty. Not only is performance of this commandment subject to reward, non-performance is subject to punishment.

2The letter ו at the beginning of the verse ואתם is a strong hint in that direction. G'd said to Noach that inasmuch as He had sworn never again to destroy all flesh, man too must perform something in recognition of G'd's oath, i.e. to be fruitful and to multiply. We can now understand the statement in Shabbat 41 that if someone takes hold of his member and ejaculates, this is tantamount to his bringing a deluge on the world. Our sages considered such action a breach of the mutual oath between G'd and man. When man deliberately destroys his semen he is in violation of that oath.