Is it cheating? No. Is it crappy and dishonorable? Of course.
Take a few days and cool off. This is as much for your good as for hers. Then confront her. You need to be more concerned about the root causes of this than the actual event. If she's grinding on other guys there is a reason. She may be unhappy with you in particular, or just marriage in general. Some people don't do well with the loss of freedom that comes with marriage.
That isn't your fault, but she has made it yours to deal with. You need to figure out what is really going on and then go from there.
I hope you can save your marriage, but this is probably not a good indicator of things to come. I wish you luck.