It sounds like your girlfriend has some kind of rape or domination fantasy, and the scene in the movie was a way for her to introduce the idea to you, specifically by suggesting that you immitate the circumstances from the film.
Have a discussion with her about why the idea turns her on so much and ask if there are any less risky scenarios than the one she suggested that she might like to try instead.
Don't get so defensive - these guys are giving you good advice. Things could go south between the two of you or who knows what else, and this would definately be the kind of thing to worry about. It would be her word against your for a fairly strange and possibly illegal thing. Even if she gives prior consent, it may very well still be illegal to engage in any kind of sexual conduct with her even while she's unconcscious. Consent is generally agreed to be a fluid situation that extends during the act of sex - if she's not awake, she technically not giving consent, even if she told you to do it. From a legal perspective, you could be get seriously fucked in this situation.
In most colleges, rape can be vailidly charged by a girl if she's intoxicated, even slightly intoxicated, even if the sex act was entirely consenting. That should be food for thought as to how these sorts of things are taken by the powers that be. If accusations are levied, the law is strongly on her side, not yours.
I'm not sure what the appeal is for her here (having an experience she won't remember in the slightest), or for you (having sex with a knocked out girl, kinda borders on a necrophilia sort of thing, doesn't sound fun to me). Get her talking because I'd bet green money she has an underlying fantasy that either involves rape, which some women actually do, or she just wants uninhibited or dominating sex of some sort and she feels this is a good introduction to participating in a fetish that she's probably not comfortable openly disucussing without a little engagement from your end.
And, as the others mentioned, the health risks here are pretty high. Imagine explaining to some paramedics that you were 'knocking out' your girlfriend for entirely 'consenting' sexual purposes when she just happened to go into resperatory failure. It takes a trained anesthestiologist to take somebody under safely, and things still go wrong sometimes. You're way our of your league and begging for trouble with this kind of thing. Besides the legal implications I'd advise against simply due to the intense health risks for her.
Frankly, I have to agree when others suggest that you're being naive about this. Mostly because you don't seem aware of any of these risks, and secondly because you claimed to be asking for advice but reacted with anger when others helpfully tried to alert you to just how unbelievably stupid and dangerous this idea is.