No, it is not. You should touch a breast everyday.
By 'cheating' I presume you mean having sexual adventures outside your present relationship. Touching someone else's breast on purpose would certainly indicate that you have an intention to cheat.
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Yes, touching a girl's breast without her consent while in a committed relationship would typically be considered cheating, as it is a violation of trust and boundaries. It is important to communicate openly and honestly with your partner about what is acceptable behavior in your relationship.