If it was just a kiss, do I have to confess and risk losing a relationship or not?

Exactly as the title in my post said. It it was kiss, it wasn't suppose to happen and don't plan on ever speaking to that girl. I hate what happened. My girlfriend is whom I want with all my heart. In fact, I've already prepared my speech on her b-day, on March 30. I want to ask her the question.

I think confession should be done if it's something more serious such as sleeping with someone else, that type of cheating that puts people at risk of an STD and you did it multiple times. Personally I wouldn't want to know if it was just kissing. I'm still guilty don't get me wrong but is it wise to just keep this to myself? We didn't do anything beyond kissing.

If it was just a kiss, do I have to confess and risk losing a relationship or not?
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