It sounds like she has more of a swinger mentality than a poly mentality. Sport fucking vs falling in love and romantic relationships. How much time is she willing to devote to such activities. 2 other partners. How much time will be taken away from other things. How much time are you willing to donate if asked ?
So now she has changed her mind form not sure about wanting to be married ? Or changed it back to wanting to be married?
And your insecurity is based on her falling in love and leaving you? Which to me sounds like a very very legitimate fear .....at least the falling in love part...there are millions of stories of swinger having very specific ground rules to guard against that and ....wooops it happens anyway. And unfortunately the leaving part happens too . I know it's not one for one but read this forum you'll see that it happens. I'm sure the swingers forums has all kinds of similar stories.