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For the US military folks, the US Uniform Code of Military Justice specifically prohibits Public Display of Affection, especially if it is between two uniformed individuals. I remember getting chewed out by an Ensign because I had my hand on my girlfriend's hand on a table. Guess he was jealous, she was one of the few women who looked great in uniform.

There are always exceptions, though. The most notable one is when a ship returns from a deployment. There is more hugging, kissing and groping going on than a Roman orgy, but nobody really minds.