@TooHotInAZ,
If you want your marriage to work, then get rid of the guy on the side and concentrate fully on your husband, preferably with counseling (either together or apart). In particular, if you have children, please at least make an effort. I don't mean that old crap about staying together for the sake of the children - I mean making one last effort for their benefit - but if it doesn't work, then at least you can say you tried.
If you want to be with this guy on the side, then get a divorce and give your husband a chance to find someone who will try love him. He's, I am guessing, around 40, 42 or so. He doesn't have to resign himself to just keeping the status quo for the next (on average) 35 - 40 years.
Keeping both of them means you don't give anyone your full attention. And by God I hope you're using birth control, given you are probably also around 40 and can still get pregnant. Unless you want to spin a big wheel to figure out who the dad is, make sure you are protecting yourself that way. I would also seriously suggest to you the need for condoms in order to protect yourself and your partners from STDs. Don't give me that crap about you're only sleeping with two guys and your guy on the side only sleeps with you and his wife. You don't know if he's got a second (or more) chippie on the side, and you don't know what his wife is doing. Bringing home gonorrhea is going to be a fun conversation, now, isn't it? So protect everyone.
Finally, of course the FWB guy is happy as a clam and doesn't want anything to change. He's got it good ten ways till Sunday. He gets to **** you if his wife says no, and he gets to keep his money and his home without her taking him to the cleaners.
But hey, you're welcome to this selfish jackass if you really want him.