Girl, you are the perfect candidate for a swinger party or club. You are what folks in the swinger community call a 'unicorn'. That rare mythical creature that few have seen... LOL
Seriously, I think that the swinger community would be a great group for you because you love sex and casual relationships.
That community is however very afraid of drama, so make sure that it's really what you want and that your legal husband is not going to cause trouble.
I'm a very strong proponent of honesty, but in this case, especially if it's just a one night thing, I see no reason to say anything. However, if someone asks, you should be honest.
I also agree with Rich in Love. Why are you still married? You just seem to be in this complicated limbo state... If it's just that no one has gotten around to the divorce, then just do it and get it done. If you still love each other, then take steps to work out your differences and get back together. If there are other issues, then I'll leave you to worry about those...