Sex deals with intimacy
is it possible perhaps that you desire to become more intimate with your partner but just can't and that you become like stone when it comes to expression?
is it possible that your unconscious mind shows a symbol of this in a dream scene featuring a co-worker or maybe someone you view as able to express what you feel?
your subconscious may be telling you it is 'co-working' with you to resolve this if this is the issue.