"Apparently the degree of force is not relevant, so if you chuck a paper plane, please expect a brick to the back of the skull, because you provoked it? This is some very dangerous thinking, and it actually angers me greatly to read these comments."
The degree of force is not clear though. OP said she felt tearful not because of the pain, but because she felt violated.
However the man felt violated too, when she felt fit to respond to his perceived snarkiness by hitting him with a newspaper, which is why he hit her back.
So both parties feel violated. OP felt pain but doesn't mention bruising, wounding, etc.
It's not clear to me that you can make this a gendered argument. If a slightly built man flicked a big burly bloke, and he responded by giving him a good thump on the arm, people would say 'what the fuck did you expect?'
The actual degree of force is completely relevant. If you went to the police and you flicked someone and they responded by shoving you and you aren't injured they would not pursue the case. The police do not investigate pushing and shoving.
If he HAD knocked her out with a brick that's a different story.
But in the real world, rather than some Mumsnet fantasy land, someone slapping you because you flicked them is not going to provoke a 'he's obviously an abuser, lock him up' sort of response.