Ok, I'm going to respond here to the outpouring of advice I received, since I know kung fu tab fu...

Some statements that jumped out at me:
Quote Originally Posted by Coidzor View Post
1. That's a ridiculous situation.
2. Why did she even need a date-date in order to go and dance at all? One can dance with friends without taking someone as a date, yes? Or even just go with her girlfriends and dance with thems as aren't attached at the pelvis to boytoys for the entirety of the dance.

3. Yeah, she's been silly, but she's probably more defensive about you seeming to be making her feel bad about having fun than actually having any ill intent.
4. Grinding is always sexual due to the fact that the genitals are stimulated, most especially in the male. If it's not all but frottage, it ain't grinding. And that's a no-no that should have been laid down from the get-go and communicated about clearly if it was to be an issue.

5. So yeah, she's defensive, you both failed to communicate about what would and would not be happening due to her dancing with others, you're upset...
It's not exactly a "go without a date" type of dance. It's sort of like prom, you can go without a date, but it is socially less acceptable.

I'm with you about the "it's always sexual" thing. She claims she can demonstrate to me the sort of non-sexual grinding that took place. We'll see. She admitted that it is, at the very least, always suggestive.

I really didn't know I had a choice in the matter of her dancing. She asked me if I would be ok with her taking him to the Sadie Hawkins dance (Christ that school has a lot of dances), and I said yes, as much as I was ok with her taking him to this dance, and the dancing that went on. I had assumed she knew I wasn't ok with it, because I thought it had come up as an issue in the past. Apparently, she hadn't. I understand somewhat better where the defensive-ness is coming from. She thought it strange I had picked now to suddenly start caring about it.

Serp:She oddly drew the same comparison, that a tango would be more worrisome. I feel, personally, that there are reasons to tango besides the sexual element, all the steps, etc. Grinding, by contrast, has no attractive elements besides the sex and the ease of the dance. Pelvises/organs were definitely touching in this example.

Ranna:She said she would be fine with this, and my best friend actually recommended the same course of action. I find myself not wanting to. I also remember what happened the last time she told me she was ok with something I suspected she wasn't (it was bad).

xPx:It's not that I worried, just that I find it inappropriate and don't want my girlfriend doing it.

To all, I have an update, I called her today and we talked some more about it, she said she was prepared to tell whoever her date to this next dance was that that was not ok. She didn't like it, but she was prepared. I explained to her that I don't even like her going to dances with other guys at all, and I think I'm already making that and other compromises in our relationship, and so I didn't think asking her to not dance in one specific way was a large thing to ask for. She agreed. She also asked me to prom, which I declined.

Thanks for the advice, guys.