Unfortunately, consent doesn't work like a 50/50 rule. If it's not 100%, it might as well be treated as 0%. As I've said before, affirmative consent is necessary, not just the absence of rejection.
Whether or not he's "tempted" doesn't affect whether or not he gave consent. And Shepard was clearly coercing him to drink further. "Glad to see you're coming around, James."
You should know that alcohol affects everyone differently, and what appears to be a couple drinks may have far more or far less alcohol than a standard drink.
And again, you've shown no examples of Shepard being at all drunk.
Why is the law ridiculous? If you're unable to think clearly, you're unable to consent to sex.
I get that this consent stuff is the law for you. But damn, if I couldn't make up my mind before and just see what the night brings, eventually giving in to my desire, then it is what it is. Maybe I'm breaking the law, maybe I regret it in the morning, but I did it because I wanted it at that point. If I need liquid courage, then I do it. I'm sure I'm breaking the law several times a day because I walk across the street while the traffic light is red. And yes, if the police sees it I'm gonna pay for it, but I knew the risk. Same with liquid courage. If I start going down that route, I did it well knowing the consequences. Unless you're only a drooling drunk that can't get out a coherent sentence anymore right before making this decision.
And I do know that alcohol affects everyone differently, but from the game we know that James is certainly no choirboy, quite the contrary. And generally, the more often you drink, the more you're able to consume before feeling its effects.
A case where Shepard's drunk? Well, if you either don't start the thing with James, or start the thing with James but don't pursue it later, she will end up with Javik (energetic party only, implying a lot of alcohol is consumed by literally everyone), clearly a case of drunk sex from both parties, which suggests she did some drinking in between. Which would also count for further pursuing James. But then, I also said Shepard doesn't necessarily need to be drunk, or sober, for that matter. It's a matter of how you roleplay her. You can have Shepard drunk, you can have her sober. Depends on the player. After you take the picture, there's a fade to black screen. Anything can happen there, anything the player wants to happen can happen there.
Okay, so what if girlfriend and boyfriend down some glasses of wine each, are they drunk according to that law then yet? If yes, are they really breaking the law by having sex after that without confirming if they both actually agree to it? When being a couple? If that's really the case, then sorry, but that is absolute bullsh*t. I don't know the specifics of this law, as I've only heard and read a tiny bit about it, but it sounded as if you could get really in trouble when you're having sex with someone while both consumed alcohol.
The problem I have with the way you presented things in here is:
0 alcohol: able to consent
Any amount of alcohol, even if little: unable to consent, because you have the druggy effect in your system already
If that's what you really mean (because it strongly came across like this; if not I'm sorry, then it was a misunderstanding)...
Then this is black and white thinking. There's a lot of gray in between, there are several different stages on how drunk you can be before really not knowing anymore what you're doing and can't make any decent decision anymore.
You said earlier that if James drinks himself into a state where he is unable to consent, then what happens to the point before? The point where he goes along with liquid courage? The point where he clearly isn't as intoxicated as later. Given that he acts on what it would take is consent for me. Or do you have to verbally verify... Hey look, I agree to having sex with you... later. Let me just down these few drinks first, ok?
I know I would say for myself I give my consent by acting on it. I know the score when I go along in order to reach that state I wanna be in to pursue things further.
And as we said before, James is conflicted, yes or no, heart or mind, but ultimately it's a yes because he keeps drinking. Then this is his responsibility. He's an adult, he knows what alcohol does, he could've told Shepard No before, had he really wanted to stop her pursuing him. Seeing as he's definitely not afraid to voice his opinions during the entire game (there are some great examples where he's pissed at her or disagrees with her), so I'm not buying this "Shepard manipulates him with her position of power". James is not intimidated by her like that, he's not afraid to speak up in front of her, or afraid of telling her she made a mistake.
And I'm also just really not buying he is so drunk at the beginning of the party already that he can't say No to her.
It´s against the law in the US to have sex when drunk?
Not making someone wasted deliberately and then having forced sex or (ab)using the opportunity? Oh my, people would go on the barricades if someone tried to implement that here. You could probably arrest half of the population during Fasching or Oktoberfest. 
To the issue at hand. No problem with alcohol and sex at the same time and the rules about fraternisation within the army are a lot looser over here.* But James doesn´t really look comfortable the whole time and it seems that he isn´t really comfortable with the result and considers it a mistake at least. And oh my, the german VO sounds like a sex hungry Ardat-Yakshi on the hunt, when she´s flirting. So I feel uncomfortable enough with that, to never go that route.
Oh haha, a fellow country man?
Maybe because our rules are a lot more loose is why I see it so different than the others. I just can't see why both my partner and I can't be drunk and have sex, happily, without ever checking if it's actually okay for both.
It's quite clear that James does consider it a mistake, yes. But that happens all the time in real life. I don't see why it can't be shown in a video game. It's not reality anyway. Especially when basically the whole Citadel party is some huge f*ckfest where it seems everybody hooks up with everybody because everybody drinks enough. If we follow the established rules everybody who's drunk and sleeps with someone is not fit to do so. And you honestly can't tell me that this is the case for let's say MShep and Tali, or FemShep and Kaidan when they're in a relationship since ME1 or ME2 in Tali's case. They're all adults. They can act responsible enough if they want to.
Honestly, in the end, I think now this is a matter of perception. I said it before that FemSheps flirting IS creepy, there's no doubt about it, and yes, it is handled very clumsily and unfortunate. But given that BW surely did not want this scene to be depicted as rape, in addition to James being no stranger at all to alcohol (it's indicated several times throughout ME3 that that man drinks), I can see it in a way that he can drink quite a lot before he actually can't think for himself anymore. And Shepard approaches him after he asked her to drink with them, too, plus this is at the very beginning of the party. There can't have been so much alcohol in such a short time. Various other characters actually start to sound drunk after 1 or 2 Glyph interactions if you keep the party energetic. Liara, Kaidan, Tali, just to name a few. Grunt sits in the shower at some point, totally wasted. James is normal the whole time, he doesn't sound drunk at all in comparison to the others. And yeah, this might be coincidence, I don't know if the voice actors actually had guidance on how to sound, but I can at least draw a parallel.
This is my personal interpretation, and you guys have yours. A lot in this game is a matter of perception, how you interpret dialogues, scenes etc. You can headcanon different stuff while other people come up with something else, and I guess that's normal. If I want to see James in a state at the beginning where he still clearly knows what he's doing and he doesn't tell Shepard NO, then I can. It's nowhere confirmed how drunk he is yet. And going from my personal experience, I can justify that. You can still think for yourself when you've consumed some drinks. The word "drunk" can mean a lot. A lot of different stages. We don't know in which state he is, so it can go either way.
And btw what I also find so damn weird is that out of all the straight romances for FemShep, only Kaidan's (and maybe Thane's, haven't seen it though) seems natural enough. She's creepy with James and Jacob, from what I heard she's pushy with Garrus, too. If she doesn't pursue James then she ends up with Javik, clearly also regretting their drunken sex...
Maybe FemShep is just generally written that way. No idea why, as it comes across as pretty weird and sad for her character, but at least she's consistent in that. Someone had a real weird thing going on in writing all these "flirty" lines for her.