I do not think we who are not in open marriages, cannot imagine them and how they work can understand them. Marriage is a social construct and I am sure there have been many ways people have related in the past (and in the present) that are hard to understand but which "make sense" to those who practice them.

I know a person can love more than one person but usually, in my world :-) that is done sequentially, not at the same time. But I can imagine doing it at the same time. I have known people dating more than one person at a time and having trouble "picking" which they wanted to be be with more, etc. so I can see not having to pick as being something some people would like.