I have those fantasies, and my wife shares them. We even thought about trying it years ago, but decided against it. I know that when we were dating in college, she had sex with several other men over a period of three years. She felt really guilty because we were committed to each other and planned to marry. I didn't find out about it until after we were married, but I found it extremely exciting. Now she tells me about her real experiences with other men, and it does not endanger our relationship.