Friends-With-Benefits Etiquette?

I can only begin to imagine the feeling of immense joy you must be feeling right now to have not only one but two genuinely attractive women upset with you. I'm not going to go on and on about my credentials, because, let's face it, I could have just made them up. Read what I have to say and if it makes sense, follow it, otherwise, don't.
First of all, DO NOT CALL HER for about two, three days. You are in a decent spot to begin a relationship with her if you want, but you CANNOT ACT LIKE IT..yet (more on that in a bit).
Second of all, there are some questions you need to ask yourself...
1) What other "benefit" contact have you had with this girl?
2) What's the girl's relationship status been since you've known her? (and before, if you know it)
3) What's your relationship status been since you've known this girl? and lastly...
4) Do you like this girl (i.e. can you see yourself on dates with this girl, etc.), or do you just like sex with this girl?

Your post said "From time to time since then, whenever we're both single and simultaneously long on lust while short on opportunities, we get together". How long are her relationships normally? How long are yours? If she's had a string of long relationships with you to provide (and I mean this in the nicest way possible) filler for her, there's a good chance you've fallen into the dreaded "friend" pit. Perfectly normal, decent guys are flung into here by girls that consider them "like a brother" (or something to that effect). Once you're in, it's very difficult to get out in a girl's mind. Fortunately, you have (since last night) exhibited qualities found in the Tao of Steve (sourced at Wikipedia):
1) Be desireless - you (hopefully) have not kept trying to push the envelope with her constantly, simply by being the "thing she can't see that's staring her right in the face"
2)Be excellent - She walked in on you making out with someone (probably, in her mind) much more attractive than her. Seeing you like that totally changed what she expected out of you, and turned you into a now-more precious commodity. All you have to do is now...
3) Be gone. I'm not saying forever, but certainly not the next day, no matter how badly you feel/think you need to. She's expecting you to, and it's going to drive her crazy when you don't. Just wait a couple days, that's all. She didn't want you to leave with her because of how embarrassed she was when she saw that she had been upstaged by some stranger.
When you do talk to her, I highly recommend telling the truth, they seem to like that. Tell her exactly what you posted: you were going to a party with a girl you wanted a relationship with, but who told you "no way". You then proceeded to find someone more "available". Based on how the events of that night went, you clearly were willing to dump the new girl for her, so it's not like you didn't care at least a little about her.
Right now your friend is a little embarrassed, a little jealous, and most likely quite a bit confused with herself right now. As long as you haven't been trying to pursue her at every turn, I suggest you call and ask to talk face to face, or to get some coffee (a stimulant, will help the conversation move along). Try to see things from her perspective, and maybe she can start to see things from yours.

Hmmm, interesting one. She's either in denial about her feelings for you, or a master manipulator (or both!) In any event, you owe her absolutely nothing.

I'm curious what ended up happening with your new friend that you apparently ditched - poor girl probably thinks she did something to alienate you. I'd be more concerned with patching things up with HER rather than your conflicted weirdo sex buddy.

I'm going to have to disagree with Amanda. Despite the age of this post, I have to comment and say it's obvious to me how she felt and it probably would have been more obvious if I had seen the whole thing. Besides, there's nothing to patch up with the woman who gave you her number.