No it is not a “rare” exception in the sense it is truly rare, however it is inarguable that one of the defining characteristics of a normal traditional marriage is the desire to build a family and it is the only union capable of doing so.
(unlike love which is a characteristic found in may different types of relationships)
The fact there are exceptions does not invalidate the rule, and to argue as much would mean that there can never be any rule’s since practically each and every single rule has an exception, however we generally rule based on the majority.