They didn't know the two were angels. The point Lot was making was that, we are to treat guests like family.
You must forgive me: I take my Scripture from the King James Version, and lack for the benefit of later interpretations. I had always taken the interest of the Sodomites to have been piqued by the unusual nature of Lot's guests: for in verse 9 it is written "And they said, Stand back. And they said again, This one fellow came in to sojourn, and he will needs be a judge" - they knew that one at least of Lot's guests was there to weigh Sodom in the balance?