Yep. At the same time I wonder what it means "nothing came of it". E.g some people don't consider kissing cheating. When the exclusivity is implied, going on a date with someone is cheating, even if you do not kiss.

It's a betrayal of trust (particularly, when others know about it - humiliating) and it's hard to rebuild trust afterwards.

Also, people are quite oblivious to how they behave until they are hit in the head. The friend's GF might dislike you but she might have been genuinely concerned for your GF, for example if you're flirting a lot with others, if you have close friends girls who like you (viva emotional cheating) etc. None of her business and she might actually be entirely wrong and evil bitch. Still some thoughts to explore. Usually, there's no smoke without fire.

Read "Not just friends" for an overview on cheating and how to rebuild trust. it might help of you want to get back with your ex (or in future)