You're in a very difficult situation and I truly feel for you, but, ultimately, my opinion on whether it's being unfaithful won't help you, because it doesn't matter. The opinion of everyone on this site doesn't matter -- even if they all agreed. Frankly, whether you, yourself, think it's being unfaithful or not ... doesn't matter.

What matters is what your husband will think when he finds out.

All that matters to that question is his answer, because he's the one who's going to have to deal with it and choose a response. If he thinks it's being unfaithful, then that's how he's going to react.

Even if you can 100% guarantee it will be kept a secret and he'll never find out, it's still his opinion that matters. If he could guarantee that you'd never find out, would it be okay with you for him to go have sex with some 18-year old? Or would that be a betrayal of your trust?

Sure, he's probably never explicitly said that he trusts you not to have an online relationship with a dominant, but should he have to or is it implied?

I know that's not the answer you're looking for, and I'm truly sorry. You want to be told it can be kept secret; it's not being unfaithful; it's okay, as long as he never finds out; that it's your needs that matter most. And I'm sure there are any number of others who'll disagree with me and tell you that.

And, again, none of that matters. What matters is:

How will he feel when he finds out and are you willing to do that to him?