I can only assume you have never been out clubbing in your life?
My partner regularly goes out without me, me without her, for hours at a time clubbing with friends and we do not go out with the intention to cheat.
Yes Sid is generally boring and withdrawn, Em likely more outgoing, but theirs is not a particularly 'unique' relationship in todays world.
It may last, it may not, but Em doesn't spend every waking moment plotting how to get laid without Sid seeing, that's for sure.