Normally, most men can finish the sexual act.
The "problem" is in how long or how good it feels, to you, and how you manage to please her.
Everyone is different. It doesn't matter if you are drinking, doing drugs, taking medicine, having sex, or anything else, we are all different. Normally, the first drink you have alters your ability to do things. The more you drink, the less it will alter things, but it will still alter things. Each drink after that, adds a little more.
If you drink every day, and sit down with a non-drinker, and each have just one drink, you will be in better shape than the non-drinker. A general rule is that every drink you have, it takes one hour to leave your body. So, if you want to be back on your game, don't plan to score until an hour or more later. Yes, this applies to DUI too.
The alcohol is a depressant. We assume a stimulant, but that is how we feel, not how it acts. So, if you think it is making you feel better, it is really numbing you, or cutting down on what you feel. This is why they used to give it to people during surgery or when injured, to kill the pain. When this happens with sex, it usually means you can go longer, because you feel less and less. However, at a point, you know you are having sex, but do not really feel it to cause a climax, and you just wither away. Basically, it shorts the circuit out. Guys need blood held in the penis to have sex, and this doesn't happen. Girls need blood to increase feeling and allow for the act, which doesn't happen thanks to the alcohol.
Is it "always" going to be less sensation? Yes. How much depends on you. A beer may do less to you than "hard" drinks, but either will cut down on fun. One drink my "loosen" her up for sex, or you, but it is the start downhill in the fun part. Of course, some guys don't care and just have sex with her, or him, even when the other person has passed out (which is where "date-rape" drugs come in at).
I mention this last bit for a simple reason. Where I live, using a drug to have sex is considered rape, even if it is alcohol. This is usually only used to harass people (real rapes aren't followed through with), but it still pays to know where the line is drawn.