Is a better question, "Does this relationship give you everything you need," regardless of whether you're mono or poly?
If not, and you ARE Poly (and the relationship is open), perhaps you can find marriage and family with someone while still maintaining your current relationship.
If not, and you're Mono (or the relationship is closed at their request), then perhaps it means moving on at some point in the future.
However, staying put with no changes if your needs aren't being met is a recipe for sadness. Be true to your own needs first.