Nothing is ever clear cut when it comes to sex. We might like it to be clear cut, but it isn't, and never will be. Sex is too tied up with very basic animal drives, the game playing is hard-wired into us.
The reality is that people communicate in non-verbal ways that are hard to write down into a criminal code.
If you did succeed in coming up with a clear cut rule people would intentionally play games with the rule itself.
At the end of the day all I can say is that anyone in an actual situation KNOWS whether the other person consents or not, whatever they are doing or saying, you KNOW.
She might be saying yes, but you might know it's because you have something over her and she's scared--that's rape. She might be saying no, but only because she's getting a kick out of watching your balls turn blue and flirting every which way from Sunday. That's a yes.
It's not simple, like relationships aren't simple, like cheating, dating, flirting, isn't simple.
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That said it is a lot more clear cut when it's paid sex, the flirting/relationship part is not there, which simplifies it a lot, and if a provider says "I don't do anal" that isn't because she gets a kick out of teasing you like a girlfriend might.
LOL, I guess a real "GFE" would be where you pay and you spend the next hour toeing the line, she says no, you say really, she says not really, you push, explore, and don't even know whether you're actually going to get laid or just fool around a little. That would be a real GFE.