Technically, you can turn them down at will and get a -1 penalty. This is not assymetrical since you can put a virtual "-1 penalty" on any AI that redlined what you wanted to request from it. The redlining of trade proposals has no impact whatsoever on gameplay because the AI only redlines items it will not trade with you no matter what you have to offer. Its only function is to make sure the player doesn't waste their time trying every possible trade combination for something they won't get anyway.
OTOH, I agree that it gets boring to have to periodically turn the AI down for the same old requests. But if any sort of red-lining were available the AI would automatically get some '-1' from time to time because it would issue automatically-rejected requests anyway. You wouldn't get rid of the penalty, you'd just avoid the annoying request message and you might even end up with an AI accumulating a lot of penalties towards you without even knowing why. It wouldn't be something I'd use too much, but for some matters it would make a lot of sense (state religion, declaring on an ally...)
Besides than that, the AI typically dogpiles on any player that has a low military power rating, especially when set on agressive stance. This depends on the actual personnality of the AI leaders, for example in vanilla Julius Ceasar is well-known for being very opportunistic about declaring and may well turn on you it he feels he can grab a couple of cities fast. Qin has also a great reputation of stabbing you in the back when you're at war with someone else even when he is 'Pleased' with you. On top of that, some AIs are easier to bribe into wars than others, and some AIs are more susceptible to actually bribe friends into war than others. I've not studied the FfH AI leaders settings, but they should work on the same principles. You could check the XML and check the settings for the leaders that declared on you in this game, that might help understand why they turned on you all at the same time.