Well, that is a pretty wild story. Bottom line is, some women will find a guy that they are just so comfortable sexually with, that they can just let themselves go when they are with them. Sex comes in different forms, making love, fucking, humping, etc... Sure, its all the same act, but it means very different things on occasion. If this girl has a drive to just have wild sex, but throughout the courtship of her now husband, felt like she couldn't show this side of herself to him, she is using you for this release. Remember, if a woman feels that a man who is courting her will make for a great long-term relationship, she will purposely hold back sex with him. But you are, instead, seen as someone who she can show her wildest sexual side to and let it come out completely. Everyone has a wild side to them. Everyone. Except, many times we hold back due to social and cultural beliefs, opinions, and biases (just re-read this thread on the mentionings of morals).
Bottom line is, she will continue to try to have these secret excursions with you because she couldn't let out this side of her to her husband in an effort to show him she was "respectful" and a good candidate for a long term relationship, and not a horny wild slutty type. Or something to that effect. In other words, she's getting something out of your encounters that she wants, and isn't getting from her husband.
Or she just loves dick.