Happy to have challenged them!
Yes, this is definitely not how men think. It's why women will talk about men as having a one-track mind. The woman's mind has a whole bunch of tracks, cris-crossing each other, going in different directions. She COULD sleep with him... or be friends with him. Or have a nice conversation with him, and never talk to him again. Or enter into a prolonged courtship with him. She isn't sure, she just knows she likes him, and would like to know more about him, and see what she feels as she gets to know him more. But the possibility is there at the outset (and is his to lose).
Women have varying levels of consciousness of how they "work" in this regard, in my experience. I have slept with girls who when I talked about it basically said they thought about it a little when I first approached, then really decided yes at some moment later. I have also slept with girls who told me they never even thought of sleeping with me until I kissed them in the apartment. Obviously, the potential was there all along with both types of girls... but some realized it; others did not.
The "vibe was platonic for so long" thing is a thing that will happen often with the less-conscious girls. They may not even realize they're considering you for sex until something you do flashes a big neon "SEX!" sign in front of their faces. And which point they say to themselves "Oh! Sex! Okay!" and they think it just popped up out of nowhere. It didn't pop out of nowhere, but because they don't have conscious access to that it feels to them like it did.
The mind's a funny thing, eh? :)