Are you saying that your husband has also been sexual with your crush? So he knows him in that way, knows about your playing, knows you two hang out in a friendly way. I think it's only natural that deeper feelings don't suddenly appear out of the blue, so I wouldn't worry that you're cheating. You seem just as surprised by this as anything!
I think it would behoove you to tell your hubs something like "I'm beginning to feel more for __ than I expected to, and it's caught me by surprise. There is a depth to our friendship that I wasn't prepared for, and I just wanted to tell you because I don't want to hide anything. We really care about each other and I hope it's okay with you that I continue seeing him, because my feelings for him haven't changed anything about how I feel for you." Or to that affect. Just keep it simple and heartfelt. You don't have anything to feel guilty about, from what you've written, but you'll feel better the sooner you tell him, I believe.
So, breathe, relax, and stop being so hard on yourself! And welcome to the Forum!