Overall, I would say that the relationship between Jonas's parents is not anything like what we would call a marriage.
To begin with, there is of course no sexual attraction between the two of them as there would be in a marriage. Once people start to have "stirrings" they take medicine to suppress them. Therefore, his parents are not attracted to each other the way married people should be.
Second, there is no romantic love going on between them either. The two of them did not meet and fall in love. They are just assigned to live together and to raise the kids to be "normal" members of their society.
So marriage in this society is much different from ours. Jonas's parents seem to get along together well, but they are more of friends rather than being in love the way married people in our society would be.