Nick is "put off" by the theatrics that take place at the dinner party.
Though he is impressed with Daisy and Jordan for all their cool and distant posturing in the heat of the day, appearing as if they are somehow inhuman - angelic - it does not take long for the negative impressions to begin to overtake these more idyllic musings.
Tom's affair is revealed at the dinner party as are Tom's racist and bigoted views of "history".
That Tom explores books on racial prejudice as a science suggests that he is shallow, supercilious, and extremely snobby. (eNotes)
Daisy states that she is so "sophisticated" in a moment full of irony, blatant performance, and arrogance. She plays the part of the cultured socialite but also clearly demonstrates a lack of actual consideration for anything intellectual or culturally oriented.
Nick takes all this in during the visit and his disappointment is rather keen. He had hoped to find actual sophisitcation in the east, not such shallow performances and uninformed, haughty postures.