I can relate to what you are saying. I love my boyfriend very much, but I also love another man. My bf is mono, and I have in the past encouraged him to see others.
If I'm honest with myself though, I know that I would be so upset if my bf was to love someone else like he loves me. I would like to think that I could come to terms with it if I needed to, but I'm far from sure that I could. This makes me feel selfish, as it means that I'm asking him to be accepting of something that I myself could not live with, if the roles were reversed.
In truth, I'm relieved that he does not want anyone else, I'm very happy that I have him to myself. I know that this is a double standard and yet I cannot help the way I feel. It does not help with the guilt that I already feel anyway.