Fetishes, by their nature, don't make sense.
I'm not particularly into this, but I agree with Cryssoberyl that the hot-and-nasty sex suits NanoFate just as well as the sweet-and-soulful kind. (And I'll admit to a certain amount of glee at the discomfort of other readers.) I think the two references to men in the book are less about flirting and more about flaunting; in a world where Nanoha has all the power she needs to back up her autonomy, she gets off on going "This is mine, not for you" for both Fate's and her own body. I don't think that's particularly in-character for the usual Nanoha, but there you go.