Well I wouldn’t think it classy to ask for the ring back if a divorce was mutual or on your request, I can see why you would want it back if she was the one that cheated. The lawyer said that she fulfilled her contract by marrying you but I disagree. I understand that you are not being guaranteed a lifelong commitment despite the vows, but I think part of marrying you is staying faithful on an ongoing basis. If she was done with being part of an implied monogamous relationship which is what marriage connotates, then she should have told you openly and then dated the other person.
The fact that she led you to believe that you were in a monogamous relationship — so that you were fulfilling your end by staying faithful but she was not - gives you grounds for asking for the ring back in my opinion. Not just for monetary reasons, though that is an incidental side benefit, but because of what it represented, as you said.