Am I weird if I like to look at pictures of pretty women sitting and standing with slouching posture? For some reason I think that women looks sexy when they are slouching.

For some strange reason people think its bad to be different that others?
While I have never given their posture a 2nd thought, I can see where your preference might indicate a preference.
Someone that is 'slouching' might give the appearance as someone that is willing to let their guard down. They are not concerned with appearances in a formal sense.
These people might seem easier to approach than the formal posturizing ones.
It gives a sense of relaxed, homely attitude.
I mean who maintains posture and formalities while relaxing in front of the tube?

On dating sites, it is normal for people to 'expect' a formal date experience. The 'slouch' would be 'open' to a more informal experience. Able to let their hair down and act naturally.

Personally, this is a trait I 'look' for in potential matches.
Never called it 'slouching' but I prefer to call it natural behavior.
I will not do the 'date' thing.
Its not natural and doesn't show you the actual prson you are dealing with.
Its better to just go do things together and live life as it comes.
Then you have a better idea of the person you are considering.
Anyway, that is what I get from your thread...