To be honest, I think Nanoha is Raging lesbian, with hints of being bi curious only with Yuuno. If anything were to happen, I think it would be this:
* Nanoha marries Yuuno ONLY IF they need to have to legally support her adoption this way, and if they don't allow civil unions on Midchilda. Fate and Nanoha would still be lovers.
* Nanoha and Fate get it on a LOT.. with a little help occasionally from Yuuno, to help the couple produce more spawn, if civil unions are allowed.
Fate, Yuuno and Nanoha have a few drunken nights that are NEVER TALKED ABOUT, even if both kids from Fate and Nanoha look disturbingly like Yuuno.
So either way, we all Win...