Some thoughts on what you just wrote. I don't think it makes sense to start an online relationship expecting it to never go farther than that. Love grows in its own ways in its own time, and has its own demands. That doesn't mean you have to go along with those demands, but you may well be setting yourself and another person up for a LOT of heartache if you say "we can be in love, but we can't be physical." It's sort of the opposite of the way some swingers go about things, "we can have sex but we can't fall in love." Either way, it just doesn't seem like a realistic mindset to me.
As for what would keep you from leaving your husband, well... love, right? The whole idea of poly is that you can love more than one person, and that a new relationship can actually make an old relationship stronger rather than weaker. Even if I didn't ever see Eric, I'd like to think that I'd make his relationship with Gia stronger because she'd be happier and when someone is happy they can give more of themselves. I would never, ever try to break them up, and hopefully any potential new flame of yours would come into things with a poly mindset of multiple-loves-are-good instead of the more traditional mindset of society at large which would have him say "she must love me and *only* me" and would be respectful of your marriage.
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.