Personally, I wouldn't be okay with a rule that says I have to call things off if I developed feelings for someone. I mean, so why am I in the lifestyle? Either I have mediocre to good sex with men that I don't have that intense chemistry with (I need that connection to put things over the top, unless it happens to be when I'm ovulating
I'm lucky- fortunately my husband understands that. We started out with the same rule, that it would be sex only, and then I found someone that I had more with. And I'll be 100% straight and say that I knew from the very first time that we met that it was going to be more, and I still went ahead and did it. I'm sure as hell not perfect, and I hurt my husband. He didn't deserve that. But he loved me, and he saw how happy I was, so his choice was to let things run their course (not to say your choice was wrong, but this is what was right for us). Eventually I broke up with him because the relationship wasn't satisfactory any more, but it was my decision to do so. As a result, I think my husband is less threatened by emotional relationships because he's seen it play out. He knows that I know that it's not "real", and that he's the one who is there for me through it all.
But as long as the communication is there, it doesn't matter what you and your wife decide as long as you're both on the same page. We learned from our experience, too. I was able to articulate that I needed to have the option for that type of relationship or this wasn't going to work for me. It took months of sitting down and talking and checking in to hammer it out, but it's been so worth it to have my needs met this way. To me, the big difference is, does having a relationship on the side check you in or out of the marriage?
That's not to say I don't have casual sex in ongoing type things; hell, I'm still fucking my very first local FB from over two years ago. But we never had that intense chemistry that made me want anything more, so this satisfied my needs. Honestly, though- unless I'm ovulating, I can take it or leave it, and my sex drive is sky high. What I really need to put me over the top is the connection. So you can pretty much track my cycle by looking at when I entertain casual partners LOL.
I hope you two are able to find a solution that works for you both. I just wonder when I see that Sam thinks she could have kept it under rein with strict limits, if she's only setting herself up for more trouble. After all, she's already shown that she could lie to you if there's something she wants to do that you won't be okay with. If she finds someone with that click... she's going to want more. And now you have the possibility of a full blown affair instead of keeping things aboveboard and out in the open.
Oh, and keep in mind I identified hardcore as MONOAMOROUS when we started... I only wanted sex and had no desire for anything more. I never thought I was capable of loving more than one person since my past is checkered with serial monogamy. It just happened, but I can't and won't go back.