The main difference is "in".
When it has to do with losing one's job, the expression is always "the sack". Apparently this once meant putting your worktools away into a sack or bag, and leaving when you were retrenched. Here's your sack. Go.
When it has to do with sex, it's "IN the sack". The "sack" is a figurative term for "bed" (also, "hit the sack" = go to bed to sleep).