@SW-User Oh yeah, depends on the form of dating, certainly on online dating sites unless it's that kind one, nobody writes "Oh by the way you're going to be my side-hoe, so best be okay with that". xD
And I can at most tell how I do it, but trying to understand how you've written the unwritten rules gives me at most a headache. o.O If anything, experience and practice suggests there's no unwritten rules, at least ones everyone follows. I've seen people do all sorts of crazy stuff.
But your question seems to specify just that sort of situation, where by whatever way you "know" they're dating multiple people and "keeping their options open". I'm not saying this is necessarily wrong if no-one is lied to or given false promises, but it's certainly not for everyone. If guys can get away with this far into the relationship, most will probably do so.
And yeah, intuition is a big thing though often wrong, and the stuff I already said about how different guys who already have a girl behave. Unfortunately a lot of that behavior also correlates with what women say they want from men, ie. confident, laid-back, not horny or jealous, giving them space, keeping busy etc. I'm not saying most of those are bad things, but they're a lot easier when someone is just an option.