I never claimed it was original.
I happen to be in a situation like the OP presents.
I mostly think about the possibility of being able to come home to her everyday.
Envisioning that scene immediately puts a spring in my step (seasoned with a little anxiety that it might not work out for some unknown reason).
Of course I assume that the torrid sex scene will follow but I don’t need to flesh out that part of the fantasy. 
Jeeze, what don’t I think about. Voice, hair, eyes, skin, scent, lips, breasts, hips, ass, feet, hands, clothes, jewelry…
The kind of hug that makes time stand still. Can’t really explain it.
Everything about her. Seriously.
How to beat the restraining order.
I guess I’m a little weird, because at that point sex is usually an abstraction, like “Well, this will happen… if all else goes well… some point in the indeterminate future. And it will be nice. But before that!..”
There’s stuff like reading together and sharing the good parts of books, poking through each other’s music libraries (both “That’s awesome!” and “What, I was sixteen when I bought that, okay?”), napping in sunbeams, playfighting, finding out who makes a better pillow, or wandering around empty streets at 3:00 AM. Food gets exchanged a lot, too. Mostly they’re “knowing each other better” things.
Thanks for the answers so far 
If you’re wondering what prompted me to ask, it’s because of a song on a CD I recently bought: “A Man and His Beautiful Wife” by Breck Alan (on the CD Kissing Rockstars). If you’d like to hear the song too, the MP3 is free for download on this page of his site. I’ve liked the song for years, but listening to it again made me wonder how much company he had for these sorts of daydreams. Some, apparently.
At the danger of repeating myself, here is a favorite quote from Citizen Kane:
“A fellow can remember a lot of things you wouldn’t think he’d remember. You take me. One day back in 1896, I was crossing over to Jersey on the ferry. And as we pulled out, there was another ferry pulling in. And on it there was a girl waiting to get off. A white dress she had on. She was carrying a white parasol. I only saw her for one second. She didn’t see me at all. But I’ll bet a month hasn’t gone by since that I hadn’t thought of that girl.”
It’s usually a flashbulb moment, which can vary from experience to experience. Sometimes it’s an image of her walking a certain way; the smell of a certain perfume; a certain toss of the hair; a way she stood. A glimpse of thigh, breast, a certain smile.
There’s usually that one particular image that keeps replaying in your head - a moment when she just happened to look just right, just perfect, to imprint herself in your memory.
For myself, it depends on how involved I am with the person (if at all).
At first it’s just the thought of what first drew my attention, her hair or smile. Then I think of how it’ll feel the next time we meet, and imagine just spending time with her, just being together and holding each other.
Then if things go well I’ll start thinking more about the last time we were together, what it felt like and what makes it so good.
I tend to save the sqelchy thoughts for people I find attractive on a more physical than emotional level 
Her face, her smile, her laugh, the sound of her voice, how great it would be to grab her ass, the shape of her calves, her delicate hands, who invented liquid soap and why, and of course her in a cheerleader outift from my highschool.
I’d agree with this. My thoughts on someone I’m infatuated with aren’t generally sexual, just general memories, an image burned into my brain, or a generalized pleasant feeling whenever he or she’s on my mind.