As a verb.
e. g. I was stun by the amazing graphics of "Battlefield 3".
"That dress is simply stunning"
In the <b>sentence</b>, "It was stunning sex." 'stunning' is appropriate but the <b>sentence</b> itself is awkward. A better suited <b>sentence</b> with the same meaning would be, "The sex was stunning."
This is how you use the word "stun" in a sentence.
Her incomparable skill was stunning.
These fields of flowers are stunning.
That bejeweled cumberbund with his tuxedo is just stunning! That tuxedo complements his stunning good looks rather nicely.
the man built a stunning device for the future
My grandfather has many stunning items from 1970's.
To me, the Canyon was a stunning place.
A ridge of fur on the cat stood straight up.
The artist created a stunning mosaic using sectile tiles that reflected light beautifully.
The mayor and all his associates were voted out of office in a stunning renunciation of his policies.