no it isn't. a marriage is a political institution to form alliances and define inheritance. in several cultures you can adopt adults or kids and they become part of inheritance, you dont need to be able to reproduce. in our culture it was about bloodlines, depending where and when in europe could be mother or father side. from mother side inheritance unfaithfulness isnt a big deal or a concept anyone cares, you know for a fact the kid is hers. from father side those concepts matter because you cannot know who the father is. you are injecting your own culture into the concept of marriage, those are separate things.
United Kingdom