If you are just asking how far can you comfortably go in real life with some sort of submissive/dominant fantasy, I would say that it really depends on a lot of different factors
No... it doesn't have to be submissive vs dominant... fantasy can include anything, including idealized expectations. For example... HDs want sex more than their LD counter parts... I think both people's sense of ideal 'sex' is way off... when does the 'ideal,' or even the fantasy, impede upon reality.
Is the sense of reality we have of our partners the problem with differing sex drives, because of our fantasies/idealizations? Does fantasy cause the angst, or does the reality? How do they get so skewed?
I have sexual fantasies I've never shared... but in not sharing them, could I be, in a sense, part of my own problem? If I do share a fantasy, and other can't/won't meet it... is my unwillingness to let go of the fantasy the cause of my unhappiness... or that my partner is actually reticent?