Ok, you say that your wife doesn't want you to have feelings for anyone else. And you say that you realize that feelings will necessarily be involved. With you two still failing to see eye to eye on this, I've gotta ask, what makes you think it's time to take the step to involve another person? If and when feelings DO come into it, will your wife feel betrayed? Will she demand that you end it? How can you put another person in that position?
I really think you guys need to sort out this difference in perspective before you start courting anyone.
Also, this -- "sleep together once, but then it has to be a threesome with my wife" -- is just pretty messed up, I'm sorry. At least you're putting it out front, so it's not like it's unethical, but... it sure as heck is unrealistic, and even kind of creepy. I mean, what if a woman you're seeing thinks she'll be fine with a one-night thing, but then she comes to really like you, even love you. She knows that the "price of admission" if she wants to be intimate with you again is to be with your wife too. Maybe she likes your wife, maybe not. Maybe she's into women at all, maybe not. But have you considered the fact that this hypothetical woman you've been dating might agree to the threesome as a way to be with you, even if she *doesn't* like your wife, even if she *doesn't* like women? I mean, I know this isn't what you're trying to do, you're just trying to make a compromise with your wife, but when you say that women who like you get one chance to be with just you, and after that it's group sex or nothing, it sounds almost like you're luring them in to a situation where they may well feel pressured into doing something they don't really want to do. Why would you set up that situation? Doesn't that seem a little off to you?
And what is the point of that rule anyway? To avoid jealousy on your wife's part? Jealousy can certainly happen in threesomes. To make things "fair"? Wouldn't "fair" be that your wife gets to pursue people who are into HER, not people who are into you? To keep "too much" emotional intimacy from developing since some people (mistakenly, in my view) believe that threesomes are less intimate?
Yes, you can feel affection for someone without feeling less love for your wife. But if you court someone, date them, kiss them, hold their hand, and then make love to them, all within the context of an emotional relationship -- don't lie to your wife and pretend it'll just be about "affection", that no love will come into it, which seems to be what she wants and expects. You can split hairs all you like, but lots of people love each other deeply without building their lives together, so to say that it would be a matter of "infatuation", not "love" just because you're not making the choice to be committed to another woman is disingenuous, in my view.
Furthermore, do you really think that when it comes down to it, when you've been dating a woman for a while and you two really like each other, and you've finally slept together, if that other woman then tells you, tearfully, that she can't bear not to see you again but that she's just not attracted to women, that you'll really be able to look her in the eyes and say "Sorry, dear, either sleep with my wife or never touch me intimately again." Ugh. I think chances are much better that, if it should ever come to that, you'd go back to your wife and renegotiate. Which means you need to renegotiate now, because you're going into this with a plan that's built to fail.
and some of the other great resources out there. Ask your wife to read them too. Explain to her that you believe that you can love -- not just feel affection for, but love -- someone else, and not love her less or dismiss your commitment to your marriage. Explain that you're not willing to put another woman in the cruel position of feeling pressured into group sex. And renegotiate. Maybe this will take a lot longer than you thought it would. Maybe it's not even really possible. But don't lie to yourself, and your wife, and other women, by pretending that you've actually reached a place where everyone is on the same page and you've set up a system that will work. If your wife won't budge, maybe you can go to bars and pick up women for one night stands, or go to swinger's clubs, but I really don't feel it's cool to "date" women that you can't have actual relationships with, for anyone involved.
Finally, I'm curious what you meant by "I was kind of shocked how strongly she came on". You said that she said that you "made her smile and won her over." That sounds to me like a pretty normal, unshocking thing for a woman who thinks she's being pursued for a relationship to say. Maybe she thinks she's finally met a man who might want to settle down. Anyway, I'm glad that you emailed her.