Of course I intend to speak about this with my girlfriend, I just need to work out the issue for myself first.
This I think is the key.
I understand how that makes guys feel, stigmas and all. I don't think I'm technically classified as having ED and I sometimes get those insecurities. I think it's horribly underappreciated how men have body issues and communication both ways on the subject is vital to coping with them.
It's a kind of chicken-and-egg thing. It's difficult to share with someone when you're so insecure, yet it's one of the best ways to help move past those insecurities.
It would also be good for your partners to understand up front what you're dealing with. If they're not respectful of your feelings then it's a good sign something else is wrong. Likewise, it would be good to make clear that it's not your arousal level that is the problem but the physiology. That can probably go a long way to dealing with uncomfortable issues with her security.
Dealing with two women wouldn't be more difficult than dealing with one if you're honest, I think. Additionally, penile-vaginal is not the only way to offer sexual gratification and orgasms. Even more exciting for you is the possibility of exploration of what DOES stimulate you in ways you can get an maintain an erection that's satisfying for you. Bodies are unique things for their goods and their bads, and there's a lot of joy in exploring your lover's body. I don't think this needs to be a bad thing at all if you approach it honestly and maturely.
You seem to be on the right track.
Best of luck to you.