Eh, i honestly have a hard time believing that you don't get why that bothered her...the idea of thinking that one could request any quid pro quo exchange of even the most mild, innocent 'sexual favor' of you for something you want is extremely demeaning and implies that you're being viewed an inferior...it also suggests a sort of entitlement on the part of the person making the joke that elevates him above the group or at least above the person he's making the joke to, and the similarity with calling someone a whore should be obvious.
That said...i'm not sure if thats really "sexist" as much as its just demeaning.
In terms of the reaction...eh...how drunk were they? It also depends i mean on what his relationship was to them (a friend, social acquaintance or stranger), and how attractive he was (its actually far more insulting if an ugly or overweight guy says it, sorry but true, because the exchange then is much more onesided, merely for the beer rather than the fun of it, and the suggestion that you would do anything with an ugly person is degrading) on the other hand if it was someone of very high status in the group that would also add additional distress. The worst would probably be if he was significantly older then, that would also justify a stronger reaction.
