Book up fast yes, but I think they are hitting a limit now. The "main" couples cruises are booked but they did some extra ones and I've gotten spam mail from SLS asking us to sign up and offering a free lifetime membership if we did.
Are they only swingers couples? No. You will find non-swingers as well, especially nudists. We met an AWESOME slightly older couple than us who we had a lot of fun with personally but they complained about all the swingers trying to fuck them.
The ports were very boring compared to the ship time. We really don't care for cruises that much because its a very "plastic" view of the world and you don't really have time to explore anywhere without fear of missing the ship. That being said we looked FORWARD to days at sea.
Still at ports most people did get off for a spell, and in a lot of them you couldn't be nude on deck. At that point you are just a normal cruise ship.