Going over the thread, I don't see anyone saying anything sexist unless you count pointing out that the context of showing Thor's butt is clearly trying to be funny while showing a shot of Natasha's butt doesn't seem to have any intention of being funny.
This really just comes down to these scenes being decided mostly by hetero men.
And, yes, a more progressive thinking would be not to do that with anybody. It's just that there's a long history of doing that with women.
Yeah, I know. Says the guy with a picture of Powergirl showing her cleavage.,