And Yuno isn't a part of her harem?
Anyway, I don't think nanoha would have a problem finding anyone if she just wanted a cock.
While they could always drop in Yuno and Nanoha getting married, or just have some new male lead marry her, it really doesn't fit with the story.
If Nanoha and Fate are just friends, that means Nanoha is married to her work, just like Yuno is pretty much married to his. Their work takes them both too far away from each other and neither would give up their job for the other id imagine.
You also have to remember that they tend to pair series protagonists with each other as it rages fans to see a side character just suddenly thrown in as the main characters love haphazardly. As such, either Nanoha will stay single, or they will go the lesbian route.