Have you asked her? What did she say?
What someone wants sexually before being in a committed relationship can sometimes be wilder and more rooted in fantasy, than what they would actually really go for if given the opportunity. (The opposite is also true. Some people find that they can explore their sexuality a lot more with a partner that they are comfortable doing that with).
Maybe she doesn't want to risk her marriage just because of a threesome. Maybe she is perfectly satisfied with you and doesn't want more. Maybe her interest was more porn-fueled than being something she herself wanted. Maybe she told you that because it seemed like a cool thing to say to impress you, the guy she started dating.
I wouldn't know specifically why. You ought to ask the wifey.
