Sforno on Genesis 16:2/ספורנו על בראשית ט״ז:ב׳

1הנה עצרני ה' מלדת, "even though G'd has promised to grant you offspring to whom I will give this land, (Genesis 12,7) He has not said that the mother of Avram's offspring would be I."

2 אולי אבנה ממנה, Sarah hoped that the jealousy which would develop within her when she saw that Hagar gave birth to a child for Avram would stimulate her own biological and sexual urges so that in due course she too would become pregnant by her husband. (based on Megillah 13)

3 שרי לקול, the Torah wants to make plain that Avram was not motivated by sexual lust to sleep with Hagar, but had only fallen in with Sarai's reasoning, hoping to assuage her feelings in the matter.