It's bat shit crazy, not even sure where any of it came from, still trying to wrap my head around it I suppose.
This is important: Girlfriend has Borderline Personality Disorder (yes, diagnosed by professionals)

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So GF has had plans to hang out with one of her friends on Sunday. I live on my own, she spends most of her time at my apartment, but still technically lives with her mother. She just recently got a new job, my work gives me a lot of time off but pays quite a bit, restricted to the weekends, so we've been spending a lot of time together recently. So, it's her intention to leave at 4PM to meet up with her buddy who gets off work at 4, my friend, we'll call him Daniel, gets off work at 4:30PM, plans to meet up over at my place and start the adventures around 5PM (couple of drinks, hit the strip, we're locals in Vegas, and night hike in Mount Charleston, nothing unusual). Well, GF's running WAY behind and I'm trying to get her out of there because I have to drive her 30 mins to her car and I don't want Daniel just sitting around at my house when we were intending to meet up at a specific time. I guess she misinterprets this at some point as "I don't want GF and Daniel to see each other" instead of "we've got specific times organized here, you're about to make me late".

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NOTE: This is important. Daniel is not a fan of GF, and they have never personally met. Daniel and me have been friends for about 12 years, and he was around when GF had a cheating episode on me with her ex. Daniel has since that moment accepted the fact that I forgave her, but really wants nothing to do with her. A very "cheaters are scum, forgive no cheaters" type persona.

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So we finally get GF out of there, I show back up to my place, Daniel's been waiting there for about 20 minutes already. We grab a couple of drinks, a little weed (yeah, I smoke every once in a while) and start watching a collection of retarded YouTube videos. About 30 minutes into this, I get a knock on my front door. I open it up and GF comes marching in with 2 of her friends (a couple as well) and introduces herself to RJ and announces a giant collection of WTF. Says that she was concerned that I was having a FUCKING GAY AFFAIR. Declares that her friends were there for emotional support, that she half expected me to be "inside him by now" and sits down and explains her entire collection of worries she was having and me and Daniel are just sitting there with our jaws dropping. Well, she ends the discussion by proclaiming that her fears have subsided and she thinks Daniel's a cool guy.

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So after getting that all out of the way, GF declares the event a "party". Her friends brought a quarter bottle of Malibu, but help themselves to our drinks and weed, and it starts.
I suppose the situation at this point was technically a double date + Daniel. GF has always had some weird behavior with double dates. It's a pattern of behavior that plays out during every double date. She sits there and tries to embarrass me and thinks it's hilarious. She'll take little digs at me every couple of minutes, correct every single thing I say, make fun of me, and it gets to the point that the other couple will usually call her out on it (they did this time as well. The girl says, "why are you so against everything he says" and the guy says, "I think you'd better quit, he looks very done). I assume it's some kind of social anxiety and it's a way to make conversation and be funny.

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She sits there, busting my balls for 2 hours, shares information about my financial situation that I had specifically instructed her to keep to herself about, and finally, sits down and tells Daniel that I "talk shit about him all the time and says that I always describe him as a low life who's going no where, who can't educate himself, who's suicidal, who's fat, ugly, a drug addict, has a 3 inch penis, and is basically no one of interest" and ends that episode by saying "but you seem like an okay person Daniel, I'm not sure why he says that kind of stuff about you" (90% of that was absolutely pulled out of a fucking hat).
After this, me and Daniel are just sitting on the couch, about as awkwardly as we could be when GF's two buddies after smoking all of our weed ask me if I'll buy them a pizza. I tell them that if everyone throws in $5, we'll get one and they declare that since they brought a quarter bottle of malibu, I owe them a pizza.
I tell them that's a bunch of baloney, they get disappointed, and the couple leave to go and FUCK IN MY BATHROOM.

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I couldn't believe it. While those two are in the bathroom, it's me, GF, and Daniel all sitting on the couch together and she seems to be convinced that everyone's having a great time. I ask her if she thinks I'm a homosexual to which she replies that it was all a big elaborate joke and she never actually worried about that at all. But...she did. She sat down with both me and Daniel and we had a big, detailed conversation about it a few minutes after she arrived. She gave no hint at all that it was a joke in the slightest. It wasn't a joke. I know it wasn't. I can tell when she's joking. She also seems totally oblivious to why I'd be upset about any of this at all. I tell her in kind words basically that I want everyone out of here now. She tells me that she gets why I'm upset and thought it'd be cool to get our groups together as a surprise because she was bored. She takes them home.
I'm just sitting there with Daniel on the couch at this point...I'm like "what the fuck was that?". "Seriously, help me analyze this shit."

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He just says that the whole "homosexuality" thing couldn't possibly have been a joke and "these are the prices you pay when you date someone who's fucking crazy". We're both so weirded out that we cut the rest of the Mount Charleston/strip activities and he takes off too.
I can't figure this shit out. I went through it just yesterday and I'm just stunned. Usually, when something weird happens, you can kind of figure out why it happened or what was going through everyone's heads but every time I try to go through this situation, it just appears convoluted, like I wasn't picking up on something or like something was missing.

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Can someone please help me break this down?? What the hell...
A few details that might be of interest that weren't included...
-I've never given her a reason to think I might be gay. We literally fuck like bunnies.
-We've been together 2 years
-I did know her two friends but not well.
-She's had weird jealous episodes in the past. For instance, GF totally isolated herself from a friend that she believed was coming on to me. Maybe I was being clueless but I saw absolutely no flirtation, overly friendly behavior, or any reason to suspect anything. My father was also a private driver for a while and many of his clients were strippers (Las Vegas living), and she had a persistent fear that "me and my father were going to gang bang a stripper". I'm not sure where these things come from.
-I've never cheated or engaged in any "questionable" behavior with ANY partner, even in past relationships.

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tl;dr: Help me Reddit, it's fucking WEIRD. Please read the story, there are to many little details that need to be understood.