As another mono husband who was surprised by non-monogamy (no, this doesn't sound like polyamory to me either), I think you've been doing an A+ job of trying to be supportive. In many (if not most) cases, an ultimatum of "I'm having sex with other people, whether you like it or not." would likely result in a divorce proceedings.
I've seen a lot of thread that talk about a partner wanted to be "poly", whether it's accepted or not. Polyamory doesn't mean married to one person and screwing another.
Big question: has your wife done any research about what polyamory is and isn't? If she hasn't, I would suggest that you support her in polyamory by strongly suggesting she start there. There are TONS of resources about good polyamory habits and polyamory pitfalls to avoid. In addition to threads here, you can also google "Intro to Polyamory", "Polyamory 101", etc.
Last thing, while you're trying to be supportive of your wife, DO NOT lose sight of being supportive of yourself.