If your bf knows that you're having a fling, and is fine with it, then I see no problem. But if he doesn't know, you're hiding it from him for a reason, and that would show a lack of trust. If you're going to do something such as this then do it, so far as I see it you're going to do it anyway but you're trying to find others who agree to make you feel better about doing something which you have already admitted blurs the borders of moral reprehensibility. You won't find many here, Rei, and your arguments have so far made little sense.