How could Lot offer his daughters to the men to save his guests?
Seforno: He expected his future sons-in-law (to whom the girls were betrothed), to come to their assistance, and a fight would ensue.
Refer to 13:10:1:3. See also Rashi later, Pasuk 33. Chochmah u'Musar (2 p.270) - in spite of his honor and wealth, he showed great humility and generosity to them, for Avraham had trained him. Why did Chazal expound to his detriment? Due to lust for wealth he separated from Avraham, and declined terribly.
It says about Lot's daughters "Lo Yad'u Ish." It says about Rivka "Ish Lo Yad'ah"!
Rosh, Moshav Zekenim (1): A man enjoys Bi'ah Lo k'Darkah, but a woman does not. Rivka was a total Besulah; no man enjoyed [Bi'ah with] her even Lo k'Darkah. Lot's daughters were Besulos k'Darkah; they had not enjoyed [Bi'ah with] men, but men had enjoyed them.
Moshav Zekenim (2): They [later] enticed their father, and they knew that they are having Bi'ah with him, but he was unaware. 1 Likewise, it says about Bnos Midyan who enticed Bnei Yisrael "Ishah Yoda'as Ish."
Hadar Zekenim: Because later they enticed their father, the verse attributes disgrace to them (they did not know).
Perhaps Moshav Zekenim merely wants to show that the enticement was totally from his daughters. Alternatively, the verse hints that now they had not yet enticed a man, but later they would.