Thanks for the responses!
The situation actually has very little to do with sex - in that, I've surprised myself.
I've discussed the topic with my current girlfriend in general terms, and I've also discussed current ex-girlfriend interactions with my current girlfriend. My gf acknowledges that polyamory makes sense and in many ways seems more emotionally prepared to handle it than I would be if the roles were reversed. She however didn't enjoy my attempts to be open about my current ex-gf interactions - no compersion here, yet, but I think I need to be more open to my current gf about how important my gf is to me. I need to be more open and honest than I already am about how she is first and foremost and is irreplaceable, but I also need to be open about my thoughts about a poly relationship before describing my ex-gf interactions so that my gf doesn't think she's going to lose me.
I've very briefly beaten around the polyamory bush with my ex-gf a couple months ago when she proposed that we see each other less because it bothers her current boyfriend. That put a strain on the open loving nature of our relationship, but after discussing things she actually started calling me to hang out more often than before, however she was occasionally oddly reserved in her interactions with me. I have not brought up the possibility of her joining my gf nor even the topic of a polyamory in general because I believe she is very conflicted and is holding on to her current boyfriend for lots of forced reasons. If she is as happy as she sometimes claims then I certainly don't want to strain her current relationship. I don't believe she'll let go of that relationship (long-distance) until after graduation when she gets to spend more time with him, and in some ways I feel as though a window of opportunity will have passed to have spent time together (all three perhaps) in a more neutral environment and more often than will be possible in the future.
In any case, I am excited for my gf to come home from her vacation in Puerto Rico and lose myself in her presence again - Wednesday! I will continue to hang out with my ex-gf whenever the interest in mutual. I do not think it is my place to prod about her current relationship and I will let her figure that out on her own unless she specifically asks for my thoughts on the subject. I will continue to strengthen my relationship with my current gf and plan for my post-graduation future, and I'll continue to strengthen my relationship with my ex-gf as time allows.
It's terribly frightening to feel any sense of control at all when on the potential brink of something as seemingly beautiful as a larger family of love however improbable. I'd like to say I should just let things happen as they may, but that's a blind man's advice. In any case, regardless of the turnout, I'm in a situation, however frightening, that makes it easy for me to feel an overwhelming appreciation for my life.
Thanks for the advice. More advice is always welcome. If anyone seems interested I'll keep you updated.
Progress and Love