Honesty and self-honesty are so incredibly important! I'm glad you have that going for you already.
In poly terms, I guess the guy and his girlfriend are the primary couple and you are a secondary relationship? I'm in a similar relationship, though in my case polyamory is well-accepted by both members of the married couple, and so is the idea of caring deeply for the secondary partners.
So. You might consider your role.
Do you want to be a considerate and supportive secondary? Do you accept that your friend's well-being and happiness depends to a large degree on his primary relationship, and that the boundaries of this relationship are mostly set by he and his girlfriend?
If those things are true, then it may be that one of your self-commandments is to do nothing that undermines or stresses the primary relationship. In that case you are friends-with-benefits with the guy and you can explore friendship deeply, but you will probably not expect the relationship to become "romantic."
(That said, I am not sure where you stop saying "friends" and start saying "lovers.")
I'm familiar with this situation, since I'm in a similar variant. I find a great deal of satisfaction in knowing that I am in some way helpful to the married couple I feel such affection for.
Do you want to gently facilitate opening this couple up to a wider understanding of the possibilities of loving more than one person at a time -- ie, polyamory? Do you think that you, as a secondary, can do that without doing harm?
If so, you may want to get to know the GF. One of the things which I've seen in my short time on this board is a great deal of angst developing around the unknown: when a spouse or GF knows nothing about the secondary partner then all kinds of fears and worries grow. When they meet and the wife finds that the GF is a really kind person with a quirky sense of humor and a loud laugh, all of a sudden those fears evaporate.
So get to know her. Become friends if that is possible.
What other possibilities can you imagine for your role? I mean, being realistic and honest with yourself. I know almost nothing about your situation, so I'll leave this to you.
I suppose the main thing is to be conscious and aware. I think, actually, that you're already accomplishing that -- what you've written makes me see you as thoughtful and careful not to do harm. Good for you!
Last edited by EugenePoet; 04-19-2010 at 06:33 PM.