According to the narrator, 'Elizabeth Bennet had been obliged, by the scarcity of gentlemen, to sit down for two dances; and during part of that time, Mr Darcy had been standing near enough for her to overhear conversation between him and Mr Bingley'. Ch. 3
Only Darcy says Lizzy is 'slighted by other men' not the narrator.
Elizabeth must've been dancing as she sat down for a couple of dances due to a scarcity of men; not because men are slighting her.
If there is a scarcity of men, as the narrator says, it is unlikely Lizzy is the only young woman sitting down.
Lizzy is the daughter of a gentleman landowner, and not of inferior status to other Meryton ladies.
Lizzy does not require Darcy to give her consequence at the Assembly.
It is not relevant whether Darcy knew of Lizzy's uncles, people at the Assembly such as Sir William Lucas had made their fortune in trade. Bingley's fortune had also come from trade.
Further, Lizzy's sister Jane dances with Darcy's friend and househost Bingley.
No man at the Assembly slighted Lizzy except Darcy by his
rude remark to Lizzy.
Darcy made no attempt to fit in at the Meryton Assembly, he remained aloof. There was no reason he couldn't dance with Lizzy, regardless of being 'not handsome enough to tempt me'.
According to earlier L&T posts, Darcy really had a duty to dance as an Assembly is afterall for socializing and there was a scarcity of gentlemen. Darcy also had a duty to his househost Bingley not to be rude to Bingleys new neighbors.