Just wanna say I think Nyc's points are good ones. Opening up to your husband having casual sex with someone he has no intention of getting serious with may well be easier than opening up to him having a connection with someone for whom he has stronger feelings to start. That's the whole premise behind the rules that many swingers set up, that sex in and if itself isn't necessarily "threatening" emotionally the way love is.
However, you do have to be prepared for the idea that her *could* fall in love with anyone with whom he's intimate... feelings are unpredictable that way. It will be his responsibility to not let anything completely blindside you, but instead to keep you updated all along the way.
Don't feel bad that this is harder for you than you thought it would be. Chances are very, very good that if you keep doing the hard work and introspection and talking and stretching, it'll all ease in time.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.