When I read your first sentence, I thought, "You go, girl! Have a good time!"

Then I read the rest.  What is your arrangement with your husband?  Do you have an open marriage?  If so, perhaps it is not necessary for you to tell him.  Although why not tell him about your fantasy so that you both have the opportunity to pursue it?

However, if you have agreed to be monogamous, you need to talk about this. 

It doesn't really matter whether you're with a man or a woman, he may still think that you've betrayed him.  Intimacy depends on trust and covering up sexual activities destroys trust.  Get it in the open and talk about what you're going to do in the future.