If both parties being high is an exclusive factor for rape then nearly every couple I know is raping each other weekly. It does depend on the context and relationship but as i said in my original comment, an established stoner couple is usually able to consent while stoned. Its not the same as getting someone stoned for the first time and taking advantage of them.

For example I'm fine having sex while I'm stoned, however i wouldn't consent to it while high on physchedlics. My partners consent and condition would of course ve important there as well. If i learned being stoned was akin to being blackout for them, i wouldn't feel comfortable or confident in their consent.

Its probably taught so that when in doubt you just don't do it.