@ new1: I think the operative word is "secretly" here. What I wonder is, if she thought it was ok, why did she do it behind your back in the first place.

And itīs not a definition problem anyway. No matter what it is - if one of the partners is not happy with it, and the other ignores his feelings, it is not a sign of her loving or even respecting you very much, is it?
The point is not: 1000 people on this page say it is cheating, so by telling her that you "prove" it actually is.
The point is, she is hurting your feelings and does not care. So if you want to discuss that with her, it would be the way to go.

And I donīt mean to sound heartless, but if she does not care about your feelings, why would you want to stay with such a person?

She is right on a legal point, meaning if you take it to court her actions will not qualify as cheating; but she hurts you on an emotional level, and that is what counts here, after all.

Kind regards and best wishes
Arria