"You can drink all you want, but just stop having fun once your drunk"
If fun for you = rape, then yes. You are not allowed to have that kind of fun once you are drunk. Absolutely.
You are encouraged to have other types of fun while drunk.
0.08 Bac (or 0.05) is choosen so your reflexes are not affected. It's VERY low.
The specific number is unimportant. I am just arguing that there is going to be some level of intoxication where your judgment starts having a significantly >0% chance of being impaired. This level could be experimentally researched (and I'm sure it has been). It may or may not be similar to the threshold for motor reflexes used for relevant laws for motor reflexes.
WHATEVER that level is, that's the point at which having sex is unacceptable.
Researchers could also determine reliable measures and indicators that correlate closely with being near or above that level, once they determine what it is -- things that would be more applicable than a breathalizer to real life social situations. These could then be practically used in real life.
And it's never, slightly tipsy.
See above clarification. The level determined using the above methods may or may not exceed what you would call "slightly tipsy"
Intentionality is a requirement in English law.
Yes,
you intended to drink alcohol. (if you didn't, and were acting irrationally after being unknowingly drugged, that would and does in actual case law change everything). This is the relevant point of intentional decision (again, in actual case law, I'm not just making this up), since everybody knows that drinking lowers their ability to make good subsequent decisions, and it is purely YOUR responsibility to control yourself within your own limits of rationality.