Yeah, actually, my uncle was in a similar position. And when my aunt found out she was devastated... I sympathize, but if you love your husband then you need to talk to him about what's going on. Honesty and clear communication are after all some, if not the most, important hallmarks of a healthy relationship.
If you cheat, then of course you're being unfaithful... I consider on-line play cheating, at least emotional cheating. And having sex with another guy... well that would definitely be cheating; unless your husband knows and is ok with it.
There is a possiblity here that you simply can't have both your husband and a bdsm relationship. In that case you need to chose. Honestly, not telling him and going behind his back is an extremely selfish thing to do... after all, this is his life too, and he's committing it to you. I sympathize with you, but I have to say, your husband is more the victim in this scenario than you are. You owe it to your spouse not to abuse his trust in you.