Every step of the way in dating and seduction, women have 'levels' you can unlock. And once you've unlocked a level, it stays unlocked.
In the back of your head, in every courtship you have, you must aim to unlock new levels.
A woman you've slept with is far easier to sleep with again than a woman you haven't slept with yet. This is because you've 'unlocked' the sex level with her. This is just one level, but look at how big a difference there is between women you have slept with and women you haven't.
With the woman you have slept with, you've done it together. She remembers doing it with you. And she won't have nearly the resistance to doing it with you again that a woman who's never been with you will.
Every man realizes this... at least in principle. And at least with women they've already closed the deal with.
It's why men invest a lot more time and energy into booty calling ex-girlfriends and former friends-with-benefits than they do random women from social circle or work they haven't slept with. She might not be sleeping with you currently, but if she's slept with you before, she's a lot more likely to come for a shag than a girl who hasn't bedded down with you yet.
This principle of 'unlocked levels' is also why you see so many women hung up on their exes, even when they know the guy isn't what they want, even when they have seemingly better guys pursuing them they could sleep with (then date)... yet they go back to the ex anyway.
It's why women you hooked up with once back in high school or college, when you run into them 10 or 15 years later, are still significantly easier to bed than any random woman you've just met, all things being equal (e.g., commitment status, etc.).
Once you've unlocked the 'sex' level with a woman, it's permanently unlocked (more or less).
That doesn't mean just because you've slept with a girl before, you definitely can get a repeat at any point in the future. Sometimes doors close.
But it does mean it's going to be a heck of a lot easier for you with her than with a girl you haven't gotten intimate with yet. The doors you've opened are a lot more likely to stay open to you than random closed doors are to open on their own.
But it isn't just sex that works this 'unlock the levels' way.