Orcas aren't monogamous. The belief that Orcas stay with one mate for life stemmed from the fact that the same male orcas travel with the same females. It has since been discovered that male orcas stay in a family pod with their mothers, sisters, and aunts for their entire lives. Females mate with males from other pods to decrease the chances of inbreeding.
Whales and dolphins don't breathe automatically like humans do. They sleep with half of their brain at a time because they need to come to the surface to breathe. If someone really tranquilized an orca like Nolan did with the needle in the harpoon, both halves of its brain would be asleep at the same time, and it would stop breathing and drown.