In this case, I think you should tell him, because I agree with ID that keeping it to yourself will undermine the relationship and always be unspoken between you. BUT I strongly strongly caution you to have this conversation from a "relationship" perspective and not a D/s one. If you think a man that loves you and cares for you is going to hear this news, put on his Dom leathers, and beat you until you don't feel guilty anymore, I think you're mistaken. You need to tell him what happened, why it happened, what you've learned and why it will never happen again. You need to be really clear that you know this hurts him, and that the reason you're sharing it with him is for the relationship and not just because you would feel better if you did. Then tell him that you accept whatever position he takes on it that will make it easier for HIM - if he wants to punish you, then you accept that, and if not, you will still learn from what happened and never repeat it. But whether he responds in a D/s manner or not, please don't think that the D/s will really allow you to atone, or to relieve your guilt. It won't.