I'm sure there are some flakes who argue with a straight face that unless someone can blow 0.001 on a breathalyzer, you're committing rape if you have sex, but come on. "Do not have sex with someone with diminished capacity" is not really a difficult concept, and a lot of people who seem to resist it pull out the "Oh, I guess if my girlfriend and I have a few beers before we have sex, we're raping each other" argument.
There's at least a few different things going on here. "Sex with someone who can't consent is rape" is, as you say, not a difficult concept. It's a tautology.
I wouldn't say the people who respond with the drunk girlfriend argument are necessarily "resisting" that statement. Most likely one or both of the parties are misunderstanding the other. The statement that "having sex with someone who's been drinking is rape" is not equivalent to the statement that "having sex with someone who's too drunk to consent is rape". The latter is qualified with the fact that the person is too drunk to consent, while the first could refer to literally anyone who's been drinking, even if drinking has not diminished their capacity.
If, on the other hand, the first party has made exactly that tautological argument and the second party makes the drunk girlfriend argument, I'd argue the second party has most likely just misinterpreted the argument, or read too far into the first party's intent, especially if they're actually engaged in debate. If not, then clearly they're wrong or don't understand the concept of consent, because, as stated before, "having sex with someone who's too drunk to consent is rape" is tautological on its face.
On the other hand, there are a few flakes who have argued that if anyone has one drink they're unable to consent, and that can rub off on arguments. If someone has told me "the sky is always red (and therefore you're evil)," and I've had to argue against that position, with the accompanying baggage of being called evil, then someone else comes along saying, "the sky is sometimes red (and that can mean you're evil)," it can be hard to not immediately jump to "the sky is usually blue!" even when that has nothing to do with their argument.