This has been the stock standard response by Kate Beckett when it comes to anything
to do with starting a relationship Castle.
Forget about looking at it from "real world" plot devices and caskett relationship strategy by Marlowe.
Let's just look at it from the Kate Beckett the character.
The first time she uses the excuse of "It's Complicated" is when Agent Jordan Shaw observes that Castle cares about her.
And from there that's the phrase she turns to when confronted about a possible relationship with her "work" husband.
What's even more confusing was the fact she HAD decided to have a relationship with Demmings and then when Castle said he was going to leave, she changed her mind and wanted to be with Castle.
What was the point of starting a relationship with Demmings if she wanted Castle?
Let's look at it from a detective point of view - what was her motive for doing it?
I know I'm rambling, but my main question is, why are things so complicated for her to get involved with Castle?