It might help to toss out all the alpha beta nonsense and recognize being of a particular gender does not mean you act or think a particular way. Each of us is an individual made up of various productive and non productive qualities.
And the one invaluable quality to our species is cooperation. Without it we'd have died off a long time ago. This isn't a situation for you to allow or disallow, as a man of any sort. It is a situation for you to decide if you want to stay or go as a person. This is only going to be harder to reason through while you're looking at it as some war of the sexes with either having some agenda against the of the other. We don't have hive mind with others of the same gender. Thinking this way is to suggest we all fit into some easy to identify box and must follow some set standard of behavior. All that smells of the pick up artist schools of thought.
The fear is often rooted in the feeling of helplessness. Your relationship has changed but you're not helpless. This isn't happening because you are ineffective at controlling your partner (or ineffective as a man for pete's sake). You're not suppose to control others in the first place. You control YOU. Your GF is another whole and separate person and not an accessory to you. She doesn't exist to validate/invalidate your masculinity and likely isn't doing anything with that in mind as thought she sat up one night and cooked up some plan mess with and test you. And neither does she control you either as an absence of your control over her or just in general. You can stay, go, or redefine your association with her. These were options you had all along and not just since she told you she was poly.