Just curious, who set up the 'requirement' to address the Captain only as Captain?
Please, tell us, where did you discover the 'requirement'?
This subject has been done to death and a search of the archives will reveal all. The bottom line is that some airlines and some cultures require it, others don't.
If you happen to have been lucky enough to have slept with one of the lovely young ladies the night before then, in all probability, if you were any good, she will call you 'Captain', and if you were really good she will call you, 'My Captain'
