I have 2 men in my life; one is my fiancee and the other my kids' father. Sex is awesome with both men. I want to know why I am starting to crave one man and not the other. When I was with my kids' dad, I cheated on him several times. But now that I'm with someone new, I feel guilty about wanting to sleep with my kids' dad again!!

  You seem to have a problem understanding the word commitment. When you were with the father of your children, you'd made a commitment to him, and yet you cheated. Now that you're going to marry someone new, there is again a commitment, so of course you should feel guilty having sex with someone who is, effectively, your "ex."  Now if you had told both men that there was no commitment, and I'm not recommending this, mind you, then you could have sex with both without cheating. But once you make a commitment to get married, you're not supposed to be having sex with anyone else, including people with whom you used to have sex.  Given your attitude, you should give serious consideration as to whether you're ready to get married because from my point of view, it's not readily apparent that you are.