Does anyone give straight answers here?
You've been given straight answers.
Lawsuits are about money. In order to be awarded money in a lawsuit, you have to be able to present information that you were somehow damaged, and the monetary amount is based on what those damages are.
You have not been financially damaged: she did not damage your property or your physical body, and she did not steal money. You have not indicated that you have suffered any losses related to this, other than the relationship.
Yes, as a single person, you can date who you want, but only on the condition that the other party agrees to the terms.
Apparently this woman was of the opinion that there should be some level of exclusivity in your relationship. Whether or not that was a reasonable expectation is beyond the point; her feelings were hurt, she felt betrayed, she showed you why she felt betrayed. If she left it at that, fine - from what you have posted, it seems that she made her point, and parted ways.
You have no case.
You have not indicated that she did anything beyond being nosy and becoming unhappy at what she found.
Let me put it this way: she didn't go trash your home, slash your tires, spray paint the neighborhood with "Chris N. is a stinking cheater", tell your employer that you've been embezzling and having an affair with the boss's spouse, rehome your cat, etc. (<--
That would be something you'd have a legal reason to complain about.)