Hi! I'm new to the board, but a lot of your post was striking close to home.
I'm a poly female in a new relationship with an unsure/probably mono/who knows at this point man. It's been about a couple of months, and in classic me-style, we have moved pretty fast. Like you, he is generally inexperienced when it comes to relationships. Whereas, I've had them streaming out of my ears since the age of 14. In fact, I've been within a week of my wedding to the father of my now 2-year old child. Big difference between that and being the third or so girl that he's dated.
For us, there was no fanfare, no "Ok. you're my boyfriend now." .. I got bored one night and changed our facebook relationship status
The ever-important medium.
Like your girl, I am in a prior relationship with my best female friend. LAM seems to have the same attitude you and many other males I've encountered have - "If it's with a girl it's okay." ... I told him I was poly, he reeled (and is still in the process of reeling, hence our joining this board for a little bit of group couples counseling, ha!) .. I told him I was in love with, and involved with a lovely little gal and he just shrugs. It seems like through his struggles with polyamory, he fails to realize he is already ENGAGING in it, by simply being with me, while I'm with her, and she's with whomever, and so on
Anyway.. my advice is basic, which is... if it's something you can handle emotionally... just go with it. That's the easiest way for me. I communicate my problems as they arise, but other than that my main goal in life is to love others and be loved and just let things happen. You will find soon enough exactly where she falls into your life and where you fall into hers and it's not entirely necessary to make this comparison between yourself and her girlfriend. Each relationship is different - and to the poly partner, each relationship may be entirely necessary. You don't want to cause resentment by comparing yourself to someone else she loves or trying to get a leg up on her or anything... as easy as this may be to do.
Hopefully, I'm making a little bit of sense. I have just woken up, my apologies!