If in your shoes I would probably have a heartfelt conversation w/my husband & let him know what was going on in my head.

I shouldn't even offer advice - compared to what everyone else is saying, I'm a very bad woman because I act on sexual thoughts (but I also have an open marriage, so think what you will about me)...

buuuut, we are at the basic level, a primate & when most primate behavior (especially bonobo chimps, who are genetically very close to us) is analyzed - monogamy just isn't natural. Having fantasies is perfectly natural, desires, wants, needs... all perfectly natural. There's no need to feel guilty or bad because you're having fantasies. They can be a lot of fun!