It is helpful to make a distinction between relationship and partnership. When they talk about being 'in' a relationship or 'having' one, what they mean is partnership.

Partnerships are the favoured relationships and are defined by their exclusivity. All partnerships are relationships but the reverse isn't true.

Relationships are almost infinite, partnerships are specific. You may only have a few partnerships, probably personal or maybe business. They are likely to be codified and rule-bound.

Partnership suggests commitment and shared agenda. Relationships are broader and cover all kinds of associations and friendships. They are as important, maybe more so than partnerships, as how they are conducted defines a life.