It's possible that the "reality" of "virtual sex" is largely in the eye of the beholder.
You're also on target in supposing that what is true of mental fantasy is probably even more directly applicable in cases of "virtual" activity.
But having said this, we feel compelled to ask-is "grounds for divorce" the point?
That's because "relationship" at any level implies a degree of emotional entanglement.
When the heart gets drawn into the web, there's a greater potential for pathology.
Insist that the two of you seek professional marital counseling together.