What do you think happens if you take a low dose of marijuana or mushrooms?
You are stipulating that ethanol [0] is the only psychoactive substance which can be properly dosed for its effects? Sorry, but you realize that's rather.. uneducated? It's about potency, tolerance, bio-compatibility and the resulting individual threshold.
Case in point: Your "one or two" drinks could be beer, booze shots, cocktails, wine and many other products with varying, often not even labeled, potency. You could drink them over the course of 2 hours or 5 minutes, all of that influences the final effects, especially how regularly you are having your "only one or two drinks".
That's why there are such things as "functioning alcoholics" or people who regularly consume high potency cannabis and thus don't even notice any effects when consuming low potency cannabis.
Your anecdata is incorrect. I can smoke two bowls (or even two 1/8ths) in the course of say 45 minutes, and I will more sober than if I had two IIPAs or two shots of tequila.
This is also generally true of hard drugs like heroin and cocaine, but here I think the addictive qualities are what makes them a social problem.