Good question. In my mind, I think of only "kings", 'cause I'd say "realeza" for both king + queen.
According to a dictionary, "realeza" means:
1 Dignidade de rei ou rainha.
2 O conjunto de reis.
3 Um ou mais membros da família real.
4 O que revela suntuosidade ou pompa; esplendor, magnificência.
Actually, when you have a group of men and at least a woman, you use the masculine form. That's why "reis" was used here. I think it is a matter of context, what sounds better to each context.
"A colher é para os reis" may refer to a king + a queen. So, we can't be categorical in this case. I'd use "a realeza" in this case, and "reis" for "os reis católicos" (maybe because here it is clear that it refers to both the king and the queen).
the way I learned it is that Para is "to", and por(pelo/pela) is through. Hope this link helps. http://www.learn-portuguese-with-rafa.com/por-and-para-in-portuguese.html