I don't really see an ultimatum in what you wrote. If she had said, "You have to accept my polyamorous situation or we're over," that would've been an ultimatum. But it sounds like you and she dated for a few months and she, for whatever reasons, chose to reveal something important to you at this point in time.
While there are many polyfolk who believe in telling others about their poly situations upfront before any dates happen, before anyone even knows whether there is a mutual attraction... there are many others who feel that it's okay to wait to tell someone until the relationship seems like it might become more serious or substantial. So, unless she outright lied to you and said she was totally single and not seeing anyone else, then, she did not necessarily do anything dishonest or wrong here by waiting to tell you, even though I am sure a number of peeps would object to waiting as long as she did.
How often do you two get together and/or communicate since you started seeing each other? I would hazard a guess that this is the point where she felt like she was getting closer to you and she wanted you to know, so you can make an informed decision on whether or not to continue seeing her. If I were you, I wouldn't write her off just yet, unless she has been dishonest with you. I would read up as much as you can about it, to learn more. There is a good amount of info at morethantwo.com and other sites - check out our Golden Nuggets forum here for links to other informational sites.
I would also ask her questions about how she and her bf practice poly, ask what kind of relationship she envisions having with you, and find out of there are any agreements between them which will affect you. But, to me, it would seem that if everything's been great so far, there is probably not going to be much problem. Definitely talk about safer sex practices, find out how many other lovers she and her bf have, and ask if they practice safer sex with all of them or if any are fluid-bonded (using no barriers). Hopefully, you've been using condoms all along, but you both need to make sure that you are both on the same page about what is safe and acceptable to you and what is not.
Last edited by nycindie; 10-23-2012 at 12:31 PM.