I would look at why you are not threatened by women, I mean, realistically, all the things you fear about her seeing men are true of women. She might just prefer sex and intimacy with women. You could talk to her and ask her if she will stop seeing men, but for lots of people, polyamory is about having the freedom to love whoever you want so it might not work for her.
As for you, you don't have to date other people. Just let her if she wants.