Well, you did nothing wrong to be honest. Rules around monogamy are only made to ensure we are committed, do not line breed, curb std's etc and are not made to satisfy the core instincts of humans much like animals. So the first thing you need to be assured of is that you did nothing wrong. Get over the guilt.
Second, on being open with your bf, I think have this conversation with him. If he reacts oddly then he is not meant for your lifestyle or interests in the long term anyways, as its clear you really enjoyed this encounter and there is no reason why you should not try this again!
cheers