Wrong use of the work "f***"

Rocky has broken his leg and his buddy bob comes over to see him.

Bob : How are you doing?

Rocky : Fine. Hey, do me a favor. Go upstairs and get me my shoes. My feet are freezing!

Bob go upstairs and sees Rocky's hot twin sisters lying on the bed.

Bob: Your brother sent me up to have sex with you two.

Twins: PROVE IT!

Bob(shouting): Hey Rocky, both of them?

Rocky(shouting back): Of course, whats the point of f***ing one?