In what capacity would a diplomat and a military dude have regular contact? Does this suggest that Starfleet provides security to Federation diplomats?
Did Sisko ever serve in security? Now that I think about it, we really don't know too much about Sisko's past. Other than his father is a chef, his mother was a prophet and his wife died at Wolf 359.
But I think Ben serving as a guard to Curzon is plausible. Perhaps, a vessel that Sisko served on early in his career devoted a majority of its time to diplomatic relations and maybe they always had a diplomat, or diplomats on board.