This is only your second post. In your first post, which is also a different thread, someone responded and asked you to clarify certain things, etc. You never answered that. It would help people help you if you did go read and answer that other thread.
Having read both of your posts and going by the information available at this time, what I feel about your situation is this:
Your husband met an attractive young lady and didn't want to "scare her off" by telling her he was married and in an open relationship. He thought he would wait until there was "something there" - mutual attraction, relationship potential, etc. So when he eventually "sprung you on her", it was an unwelcome surprise.
Since that time, stuff has happened that suggests to me you do not trust him as much as you originally thought you did. You have a bad gut-feeling because if he was hiding something that important from her about your relationship with him, what might he not be sharing with you regarding his relationship with her? This is where the root of your jealousy lies. It actually isn't jealousy at all - it's your body's way of telling you that something isn't right with this. I would need you to answer the questions in the other thread and provide additional information before my crystal ball will give me any more conclusions to jump to about HIM.
Now, as far as SHE is concerned - I'm guessing that she thought she met a nice guy who is potential husband-provider-breeding material, becomes smitten, and "oh-by-the-way-i-have-this-wife-you-should-meet-you-two-would-like-each-other-you're-both-bi-what-do-you-mean-you-thought-i-was-single? I-never-told-you-i-was-single." Etc. So she cries and yells and beats him with a pillow, and he reassures her that it's ok, this can work, just meet my wife and give each other a chance. Then they have like, the most mind-blowing post-fight secks you evar did see, and as the endorphins are wearing off and he goes to the bathroom to flush the condom, she tells herself, "He wouldn't be with ME if he really loved his wife that much. I'll just wait for him to realize how much more awesome I am, and then he'll be ready to leave her and be with me instead."
The rest, as they say, is history - or more like, history in the making.
To all my biggest fans who are reading this - I'm sure you realize that I don't know what the hell I'm talking about, I'm making lots of assumptions, jumping to tons of conclusions, and totally judging people who are not here to give their side of the story. Let's see how right (or wrong) I am. Start placing your bets...