That depends on how you personally define cheating. Tough one. She didn't just share a quick kiss with another guy, but actually went out on a date with him. However, what we don't know is whether or not you all had ageed to date exclusively after only 3 weeks. Sometimes it is just assumed by one party or both. This is hard to answer without more information. How did you find out about it? Did she tell you or did you find out some other way? Either way, what she did wasn't very nice, but it could have been worse. Since she only did it one time (presumably) and it was early in your relationship. . .let it go. Hopefully, she won't do it again. Hope this helps.

if you and her were in a relationship when she dated this guy and kissed him and you just found out about it.....it is cheating

If you had told each other you wouldn't see other people its cheating - if you had in no way declared your monogomy then it might not technically be cheating but it doesn't stop it hurting. I think you should eb asking the question whether or not you can forgive her not whether or not it was cheating. If its unacceptable to you then it will feel like cheating.

You know in your Heart of Hearts it went way further than that.

You are not married. Its still a free country. She apparently is not your girlfriend. This will continue. Look for someone else.

In short answer NO. However if you already talking about the future together as a couple eg planning outings and other things as a couple- than it is. Why? Because she's already accepted your company as a partner. Who asks their sweetheart these days "_____, will you go out with me?" like back in the old highschool days? It's more of a transition these days. In the old days it was called Courting. Now it's tasting the menu before accepting the bill! Question, Have you openly discussed this with her? If she gives you the "your being jelous and insecure" speech than she DID do domething wrong. If she's open to talk, then she's being honest and you can move forward. SHame you had to find out from someone else. Trust would be crushed abit. Thats why you both need to TALK! Goodluck! WE can all give advice about other relationships. Funny though how we struggle with our own! I'm talking from experience;)

Whether or not it's cheating isn't important, what's important is did it go further, how into it were you two at that point, has she grown more in love with you since then, etc. If she seems like she's content with you now, and has nothing to do with the guy, and she's worth the risk of heartbreak, keep her around. Only you can feel whether or not she'll cheat. Look around and try to find out if she's cheated in the past. They tend to do it again.

IN A WAY YES , ,IF AS SOMEONE ABOVE SAYS THAT YOU AGREED TO EXCLUSIVE DATING OR OPEN DATING.THAT WOULD MAKE A DIFFERENCE. ANOTHER IS IF SHE CONTINUIES THIS LINE OF THING OR NOT. IT MAY HAVE BEEN A 1 TIME THING TO LET SOMEONE DOWN EASY AND TO LET THEM KNOW SHE WAS SEEING SOMEONE AT THE TIME. ALSO IS SHE AWARE OF YOU KNOWING OF THE DATE AND THE KISS. HAS SHE MENTIONED IT IN PASSING OR HAS SHE SAID ANYTHING AT ALL ABOUT IT. IF NOT YOU MAY IN A NON THREATENING WAY APPROACH THE SUBJECT TO SEE WHAT IT WAS ALL ABOUT. AND FIND OUT EXACTLY WERE YOU STAND IN THE RELATIONSHIP WITH HER. DID SHE TELL YOU ABOUT IT (YOU SAID YOU RECENTLY FOUND OUT ABOUT IT) OR SOMEONE ELSE TELL YOU ABOUT IT WITH NO HINT FROM HER . START OUT FIRST BY DETERMINING WHAT KIND OF RELATIONSHIP SHE BELIEVES THE TWO OF YOU ARE IN . ANYTHING THAT MAY QUALIFY AS TO WHAT EXTENT IT MAY INVOLVE . THEN AND ONLY THEN IF IT IS SUPPOSED TO BE A CLOSED (EXCLUSIVE) RELATIONSHIP BY HER ANSWERS THEN APPROACH WHAT YOU'VE FOUND OUT (AGAIN IN A NON THREATENING MANNER) THIS IS THE DILICATE AND SENSITIVE AREA OF FINDING OUT THE TRUTH ESPECIALLY IF SHE HASN'T TOLD YOU ABOUT IT. . HOWEVER AFTER 10 MONTHS OF DATING HER AND THIS ONLY TOOK PLACE AFTER 3 WEEKS IT COULD BE AS I SAID ABOVE JUST LETTING A FORMER SUITOR DOWN EASY . SO BE UNDERSTANDING IF THATS POSSIBLE UNDER THE CURCUMSTANCES.

If you had not had a mutual discussed and decided that you were exclusive then no it is not cheating. It is not nice, or a good thing for any person to do because clearly she was not up front with you. However, if you two had not decided that you were not dating anyone else then no it is not cheating.