Well, rape isn't subjective if someone sticks their penis in me, and I didn't want it, I have been raped.
From the other side, if I have any cause to doubt that someone might not be in a fit state of mind to consent, then I shouldn't have sex with them. If I go ahead and have sex with someone when they are drunk, then I am placing myself at risk that they didn't actually want to have sex with me, and in that case, I would be a rapist.
It really is very, very easy.
Just like, as a driver, if I'm slightly concerned I've drunk more than I should, I don't drive (actually, I probably wouldn't drive after drinking any, because I would be driving my kids who are so valuable to me I couldn't risk it) - perhaps that's a better rule. If you have been drinking, and you're worried that your decision making may be impaired, then don't put your penis in anyone.
And from the other side, if I have concerns over how much a potential lift home has drunk, I don't get in the car with them, even if I'm quite drunk myself.
See, easy. If you are worried you are unable to make sensible decisions, don't get in the drivers seat of a car, and don't put your penis in anyone. If you are worried that the person you are with is impaired, don't get in a car with them driving, and don't put your penis in them.