I don't understand this whole "threat to marriage" concept. Does marriage even exist as an independent concept? I look around at all the married folks I know (including myself) and nobody seems to have exactly the same relationship, despite the unity of the label.

No one else's relationship is a threat to *my* marriage. We bring our own threats in the door and we deal with them ourselves. There's nothing the guy next door does or doesn't do with his wife or husband that makes any affect on -- or threat to -- my marriage.