After 5 years on ships - I don't think I've seen anyone other than Filipinos or Nepalese people in security guard positions. The head security officer is usually a Brit . . . . and almost always ex-Navy. This might be different for smaller or regional cruise lines?
But to the married question - generally if you're hired in the same department, you may be cabin'ed together as each division gets an allotment of cabins. If you're in different departments, it's more difficult, but not impossible. If one of the couple has a position that rates a single occupancy cabin, then not really an issue. Don't forget that most two person cabins have very, very narrow bunk beds!
I don't think I've ever seen a security guard 'off' in port. Port time, I would think, is the time when security is needed the most and . . . 'all hands on deck'
From what I've seen, the most 'couple-friendly' departments on board are casinos and entertainment