I think the thought of my girl getting stroked from the back while she's giving me head turns me on
What's so exciting about that for you?
It's because it's just something she wouldn't do in real life.
There's nothing wrong with imagining that every once in a while.  Having fantasies about your SO is totally normal, and I'd wager she'd be happier knowing you had them about her rather than someone else!
So you realize how much it is a turn for me now.
I believe this is one of the things a healthy relationship has in it. No thoughts about cheating or getting too rough. This relationship of yours will go far, it has for me when I have had fantasies of this kind.