I would see it as wrong whereas others wouldnt, however, the fact that you were "with" your ex must mean you Should have had feelings for her.
True feelings and getting it on with the friend just dont click and is flatout dirty.

All I can say is...you did a good thing by breaking up with her.
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(other half of me says)

You're young and need to live life the way you want to live it! Don't be scared of anything and fuck those who don't agree haha.
What you did was natural...sometimes us guys tend to think with our dicks :thumbsup: