I can only speak for myself, that if the story, plot etc are not engaging enough, I will walk away from it and not think twice because I got board.
You can have some of the most intense sex in the top 10 stories, but if I dont care about the characters, why its so important that they have the sex, its just another Tuesday to me, its nothing special.
BUT if they were both long time in love, did everything to keep away from each other only to be constantly drawn back until they admit their feelings that leads to the sex, I will probably stay and read more.
but thats just me.