I'll probably get downvoted for this, but honest question - How come that ASUS tweet message was considered so sexist? Using attractive women in advertisements

I'll probably get downvoted for this, but honest question - How come that ASUS tweet message was considered so sexist? Using attractive women in advertisements has been around since the dawn of, well, advertisements. And she definitely had a nice rear. I mean what if the woman was replaced with a strong muscle man and the tweet said "Nice biceps" or "Nice pecs" or something? Hell, say "Nice rear" for a man, why not? Would that also be sexist? Why are we all so uptight about this stuff?