Re: "were kept together, to pair up with and were made up of"
They each mean something quite different.
"were kept together" means that already established pairs were not split up. The antonym would be "were split up".
"to pair up" means that individuals were asked to find different partners to form new pairs. There's no specific antonym.
"were made up of" refers to the common attributes or qualities of each individual in a pair. In this case it was people in a relationship, but it might have been, for example, people with red hair. Again, there's no specific antonym.
I'm not a teacher of English, but I have spoken it for (almost) all of my life....