I have 2 partners; a husband and a boyfriend. Both are very significant parts of my life in different ways. My husband is my rock, my anchor, we've been together for 14 years, have a child together, share debt, have shared experiences that I don't have with my boyfriend. But man was it hard being stationed 2,000 miles away from home with few friends when he was sent to the field for a month or 2, especially after we had our son.
My boyfriend and I have a different but equally awesome connection. He just gets how I think. He understand why things set me off and is an amazing sounding board, which has helped understand myself better and communicate my needs better. We are only a little under three years into our relationship, but have known each other about 5 years. I couldn't imagine my life without him in it in some significant way any more than I could imagine living without my husband.
People are social creatures, they need connections with multiple people to thrive, be they friends, lovers, or family. And if anything else, I'd say that our most significant connection should be that connection with ourselves or our children.