"About 90% of human sexual relations could be classified as sexual assault by that definition."
90% of sexual relations are the result of the introduction of substances designed to facilitate non-consensual sex throw them the impairment of judgement or the ability to fight back? Really?
"Because the point of using the drugs is to mitigate resistance.
...your link lists recreational drugs as date rape drugs (the Wikipage). It supports my definition, not yours...
"If they're using a substance that changes their state of mind how could you argue otherwise?"
Well then, sex with someone unable to consent is sexual assault, surely.