Nick is niether and yet both
I think that everyone is missing the point that Fitzgerald is trying to impress. He was writing about the decline and fall of the 2nd Roman empire as he saw post war America. What he is showing with Nick's seemingly easy going way in regards to his sexuality i.e. straight, gay or bi; was to convey the air of total hedonism that existed is the Jazz Society of the period. Nicks brief affair with the effeminate man from the party(as FSF described him) was no different than his affair with the woman from his firm or the eluded to fiance back west; it suited his wants at the time.