I guess I can relate to this story from the wife's point of view. My H's GF is mono. I know if she started having feelings for someone else then maybe she would leave him too. That would break both of our hearts. I know with out a shadow of a doubt that you can love more than one person at a time. I know this because I have 4 children and a husband that I love more than life it's self. I know without question that my H loves me and always will. I also know that he loves his gf too. I can see it when they talk to each other when you touch and when they kiss. It is possible to love more than one person at a time.
I can see where they wouldn't want you to leave. You are a part of their family now and losing you could hurt everyone. However you must stay true to what you want. I would hate to see you throw away a good thing because you can't open your mind and heart enough to see the possibilities. You also don't want to be in a relationship of any kind that isn't worth the work and effort it takes to make a relationship work properly. If you are not happy with him then what is the point?