Commander Norton has two wives.
It is not polyamory, as the two wives are not married to each other. It is, rather, a very futuristic form of polygamy, with a wife and children on more than one planet. The two families are on good terms, though they will most likely never meet - especially since the Martian wife cannot tolerate the higher gravity of Earth.
It is not mentioned whether or not female spacers follow a similar sort of polyandry.
Anyway, Commander Norton is recording a message for his two families, talking about Rama.
It has taken the team two days, as they have been very careful and checked everything at least a dozen times, but they are ready to enter the ship. They are down to what they believe is the third and last airlock. They are waiting for the final go ahead from Earth before opening the airlock.
The builders of Rama did everything in threes. Whether this is a cultural artifact, the reader does not know.
Norton has been thinking of archaeologists working on Egyptian tombs, sticking their heads into the burial chamber for the first time and finding unimaginable treasures. He wonders if Rama is a tomb, as there are no signs of life.
It looks as if tomorrow would be the day the crew enters Rama. Norton wonders if this will be the last message he will ever be able to send to his families.
Now then, speaking from the point of view as a spouse and a parent, I wonder why anyone would enter a poly marriage with a spacer.
Wouldn't they be gone for huge periods of time - perhaps even years at a time? Doing very dangerous work? And in those years you wouldn't you be alone, trying to raise your children without their help? And while they are off with other spouses on other planets, would you be expected to be celebate? That would hardly be fair. And yet, how many people would want to be your second spouse, to live with you and help raise your children, knowing that the spacer will return from time to time and would be getting bedroom rights as a spouse and parental rights over the children the second spouse is supporting and raising? It just all sounds as if it would be ...very messy.