Yes kissing is a sin and you should feel convicted of it! How dare you smooch another!.....jk
No it's not a sin but more of what you define as your personal boundaries. I have never kissed another girl (except mum/sis on the cheek) if I have to be away for a few days. My personal preference is I'm saving my lip-on-lip kiss for my future wife. I don't have issue with receiving kisses from grandmas etc on the cheek but I generally don't give out kisses (don't know maybe I'm not that affectionate?
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A good general rule is if your unmarried it's best not to kiss the opposite sex in a bf/gf relationship. Because you are initiating physical contact beyond holding hands. Before anyone goes crazy I'm not saying kissing = sex. But it is a degree/stage above holding hands and is an intimate affection/interaction. Like some posters have said this can lead to sex if people do not have self-control. IMO it's best to save your best kiss for your future spouse. Almost like a "sacred" kiss if you will