Why does he "already" call you a cheater?
Have you cheated on him before? What does he mean when he says that?
(I wouldn't stay with a guy who accused me of cheating when I haven't, but that's me...)

I don't recommend keeping secrets from your BF. If you aren't comfortable talking about how you think and feel, then maybe you two aren't ready/right together. Every time I tried to keep secrets, it ALWAYS came out, and was even more damaging that if I'd just owned up in the first place. Like...Not ONLY had I slept with someone else, I'd LIED about it, too.

You might have to make some decisions about this. Most of us don't get to "have our cake and eat it, too". This isn't cake. These are real people and they/we may get hurt if we don't think about what we're doing. You may have to re-evaluate your relationship. If you want to play with girls, then that relationship may have to change. If you want to stay in your current relationship, you may have to stop playing. If you are unsure about your current relationship, then maybe it isn't right for you yet, girls or no girls.