After Yaakov Avinu married whom he thought was Rachel, the Torah tells us, "And it was inthe morning and behold she was Leah" (Bereishis29, 25). Rashi notes that at night he was not able to distinguish that she was Leah. He explains that Yaakov gave Rachel signs, lest Lavan try to switch her with another girl. When Rachel realized that Lavan was planning to substitute Leah for her, Rachel gave over the signs to Leah so that she not be embarrassed.
Rav Wolbe (Shiurei Chumash) suggests that we take a minute to appreciate Rachel's greatness. She knew good and well that Yaakov would father all the shevatim and hence the entire Jewish Nation. Additionally, she had no idea if Lavan would also agree to marry her off to Yaakov, nor if Yaakov would agree to marry another wife. By giving over the signs to her sister, she was in essence giving up an eternity that was rightfully hers. She was giving up the chance to mother the chosen nation: the purpose behind the creation of the world.
Although Chazal tell us that it is better for one to throw himself into a fiery furnace than to embarrass his friend in public, this refers to one who actively embarrasses his friend. This was not the case with Rachel, since even with her playing a passive role Leah would have been embarrassed. Therefore, Rachel was in no way obligated to do what she did, and with superhuman strength she overcame all her emotions and saved her sister from disgrace. While she understood the enormity of the significance of giving birth to the Jewish Nation, she also understood that it is impossible to build upon someone else's humiliation.
Later in this parsha we find a similar level of consideration on Leah's part. "Afterwards she [Leah] had a child and named her Dinah" (ibid. 30, 21). Rashi tells us that Dinah was so named because Leah, so to speak, made a "judgment" (din) with herself. She calculated that if she would have a seventh son, Rachel would not even have as many sons as each of the two maidservants, so she prayed to Hashem, and He changed the child into a daughter. Leah wanted to mother as many shevatim as possible. However, since her intentions were entirely l'sheim shamayim, when this desire would have contributed to someone else's embarrassment, she did everything in her power to prevent her ambition from coming to fruition.
The extent of one's actions l'sheim shamayim can be defined by how he acts when his actions might hurt another person's feelings. If he refrains from his course of action, he has not only proven that he acts truly for the sake of Heaven, he has emulated our matriarchs in conquering his negative middos in a superhuman fashion!