Hi noob - I'm mono, my husband has been in a poly relationship with a married coworker for about a year now. Mono people do have a certain way of looking at things, just as poly people do. I can't understand the feeling of loving two people at once - it's never happened to me. If I wanted to, I could find a boyfriend for myself. I am not really interested in doing that because I don't want to take the chance of falling more in love with the other person. I don't want to take the chance - because I have no way of knowing how I will feel when it happens. My marriage is too important to me for me to even take the chance. When your mono - it's hard to understand how a poly feels. All I can say is that it's been a year, and I am only now able to get a handle on my concerns. My husband has had to prove to me again and again that he is here to stay and that having a girlfriend does not take away from me. My husband has a line that he must have used a million times in this past year which is "in addition to, not instead of."
Also, my husbands girlfriend has been in a monogamous marriage for 20 years and has a few kids. She did not go into this looking for a boyfriend or to fall in love - it just happened. However, she would leave her husband for mine in a heartbeat. At the beginning of their relationship - she would not have considered being able to do that - things have changed in a year. You just never know - and that is what is so scary for us mono's.
Be patient, be understanding, be consistent, be trustworthy, be honest. Do not belittle his concerns, instead talk to him about it as much as he needs. It will take a lot of repetition. It's simply the fear of the unknown and the what if's. Patience and communication are key.
Good luck - Kat