So the girl you've wanted for what seems like forever has admitted she wants to do you. You both have partners. She's up for it anyway.

I'm not looking for moralizing here, but she's basically said I'll do WHATEVER you want.

So do I just do her like an animal, setup fantasies I've never got to carry out or ask her what she wants?

I don't know your situation, but ...
You're obviously interested so tell her / show her you are

Before it gets out of control, just say hey let's agree on an exit strategy if a partner gets too suspicious or we feel like it's too much, okay?

Sounds like you are willing to do what she wants, at least for openers, so that's what's most likely to make her feel like it was a great idea.

You can move into what you want for seconds, hope she's worth waiting for.

Nice problem to have, but admittedly a tough one.

I guess I'd have to know the answer to a few more questions. Start with this one: Is she just horny, or is she dangerous too?

And then some of these: Does she make your heart pound, or only your cock? Do you intend to remain with your partner, or is there a chance she could be The One to spend the rest of your life with? What if something goes south between the two of you? Will she be cool, or will she go Glenn Close and boil your pet rabbit? (See "Fatal Attraction." )

I don't have the answer, but I do think you should "war game" out what could happen. If you're left with enough happy scenarios, go for it.

Does she make ur heart pound or only ur cock? Both.

I think this one swings it. No we've been cool for some considerable time...

I love the idea of war gaming it out though.

If it were me, no. I'd probably have to laugh with/at her and say she's got some great timing.

Here self interest and what is the right thing happen to intersect. You will hugely increase the likelihood of this being more than a short term thing if you ask what she wants. You win, and you've been a good guy, too.