Put it this way:
Right after announcing their 'official status' in the garden, Sarah then met her friend (or P.A.?) in the vault task. You think she'd be full of happiness at having just got officially hitched right?
But what was Sarah's reaction when her friend asked her about Chad?
Sarah said 'yeah well IT'S NOT REAL IN HERE'
Far from being happy about it: she immediately started backtracking!
So if SARAH herself is not even happy, excited or positive about it - from the START - then what chance is there?
I think we can all see what's happening here:
Sarah is using Chad as a security blanket/human shield for her time in the house.
Chad is using Sarah as a reality TV 'wave' to surf on (after fancying Marissa first, then Amelia, he finally washed up on Sarah's shores)
The whole thing is BS