First of all, welcome! It sounds like you've been very understanding so good for you.
On to the subject at hand, if you aren't jealous or envious then what's the problem? You seem to be nervous that she may leave you. If you're worried about it then figure out why. Confront her about it but make sure she understands you want it to work and you are just apprehensive. This is new to you. It's not to her. She needs to help you through it.
I'm not sure how I would feel with the situation being one sided but if you are mono and you're not concerned about it then good for you. Do what makes you happy. Remember, each of you brings something different to the table. Her girlfriend isn't better or worse. She's just different.
"Regret for the things we did can be tempered by time; it is the regret for the things we did not do that is inconsolable." - Sydney Smith
Kyle: 27 year old male
Katie (rymmare): 25 year old female
Kids: girl: 5 years old, boy: 3 years old