She was horny and wanted to have sex, you should have seized the opportunity and placed her hand on your semi-erect manhood.
...and then asked, "Does this look brown to you?"
On the serious side, I'd have dropped her like a stone there and then. It would be interesting to know how old you two are, as she certainly doesn't sound of a mature enough age (or mentality) to even be considering marriage.
My wife and I both have an agreement that an action like this, even when drunk (as it's no excuse whatsoever - you do that to yourself), is an instant deal-breaker. There will be no discussion.
The fact that she TOLD you before she went and did it, that she was going to intentionally sexually gratify another man smacks of a complete lack of respect or regard for your feelings or intentions. It absolutely disgusts me.
You say that just before that she told you to go away and dance, didn't she? Well, what if you did? Ten minutes later you realise you can't see her anywhere - 20 minutes later you find her with some guy ***** deep in her face. Sounds like exactly the same situation as my ex - "I love you", "You're my soul mate" etc. etc. A bit of drink later and she's off somewhere with another bloke. Like a lead balloon, mate.
If you choose to give her another chance and stick with it, then power to you - at the end of the day it's your decision. Someone mentioned previously that you could use this against her every time you have an argument, and that's very true in a sad and hurtful way. You WILL use it, and it'll open the wounds for both of you every time. Not a good basis for a marriage.
She did the same to her last boyfriend, and now she's done it to you. She acted like a complete slapper and being drunk isn't any excuse for it. At the very least call off the engagement for now until you both sort it out, but don't be rushing into anything now. If she wanted to "test" you, she could have done it well before your engagement party. It's just disrespectful and disgusting behaviour.