I'm afraid that this is quite common, it's rare to find someone with equal interest for two people like that. In this case, she was also straight, no doubt she tried it because she cared about your husband, but it was doomed to fail.
But we both agree that the feelings have to be mutual so no one gets hurt.
Didn't you end up getting hurt specifically because of this rule, though? Without it, you wouldn't have expected anything from the other woman in the first place, and wouldn't have felt rejected.
There are successful triads, though, but it seems they happen organically and aren't forced... Either when you meet someone you both like and notice the attraction and chemistry and ask them out... Or when one partner finds someone who later develops a bond with the other partner.
In the end, the breakup between the two of you doesn't take away the rejection you felt, and the two of them have to get over the relationship, too. It seems to me everyone got hurt.
I think what you and your husband need to do is have a talk again... but try to avoid rules about emotions. You can't control emotions. Especially a third person's. How do you say "you have to love so and so" or "you're not allowed to love so and so"? How is it realistic? It's better to have a plan B. Your plan B seems to have been breaking up the relationship, but if that proves to cause hurt and resentment too, you might want to think of something else.
Personally, I'm on the school of dealing with the partners as they come, and not deciding how they'll be, what they'll want and what they'll expect first. Of course, it easier for me in a way, because we're both straight, and so a triad would be impossible even if we wanted one. So it's not like we had a choice. But still, I have found that assumptions about what the relationship with the new partners would be like have proved to accomplish nothing good and a lot of bad.
I'm really sorry that you got hurt, and I hope that you'll feel better soon. I don't think it means anything about you, though, if she was straight, she was straight. I hope you still feel loved by your husband. I suggest you both take a break from dating and work on your relationship to make sure it's okay before giving it another try.