Karin, that is a really good, complete response!
I tend to agree - if we take away any and all persuasion, it would be tragic. A loud and clear 'No' and the other person is still trying to change the other person's mind (and ESPECIALLY after a few more 'NO!!!!'s', each louder and stronger later), is the loud and clear boundary in my opinion that that has just moved to coercion and rape, even if the other person then say yes to get rid of the someone. In this case, it would be wise to rather not do it.
Fragile state of mind is a really tricky one. For one, how would the other person know that? So definitely more of the 'grey-er' areas.
I think "Yes means yes" is THE best solution to the problem instead of "No means no". Where I think it (can go) goes wrong is:
Is it California that now has something in place where two parties need to sign a mutually agree upon piece of paper (a contract of some sorts) these days? And I'm not sure if it includes which positions are allowed, where you can and can't touch the other person, etc. Apart from by the time you're done with writing and signing the bloody thing, any notion of romance and sex are probably killed dead-dead (it will take some serious skill to get back in the mood!), if anyone change their mind mid-way through for good reason (e.g. anal or rough sex with which you were clear upfront you're not comfortable with and you're saying so), if the other person then doesn't stop, it's rape according to me. But now the other part has a signed and agreed document that proves it's consensual .... Murky, murky waters!
I suspect the "Yes means yes" approach is maybe working too well for some (feminists mostly). I've been seeing, reading, and hearing a lot of noise about consensual sex where the woman cries rape the next day leaving the guy baffled. The reason is, and I paraphrase, 'the sex was bad and in the clear light of day I changed my mind'. Not before or during sex, THE NEXT DAY. Some (feminists mostly) are pushing for this to be categorized as rape too.
I suspect porn and the higher prevalence of one-night stands/first date sex/sex within 1-3 months even are probably contributing to this issue more than anything else. Gone are the days where couples took the time to get to know each other and each other's bodies in various ways without actual penetration ... Knowing before you sleep with a guy whether he really is gentle and caring because he has shown you it over time many times over. Knowing before you sleep with a woman whether she has had any bad experiences, her likes and dislikes, her body language ... It takes away so much of these 'modern' day problems IMO. But I digress.
Like you say, this is a complex issue. Now bring in the individual aspect where we all had different experiences and exposure to the other sex, and it gets real complicated really quick.