Lots of thoughts here that all could be really useful. Here is my .02 cents.
I think that you need to put the poly aside for now and really work on this. This means no other partners, just the two of you. This has really gotten off track it seems.
I would suggest he find out what the rash is, get something for it or at least a course of action and start using condoms to protect you. This if it is not happening already.
Then I think you would have a romantic date once a week for as long as it takes. No pressure to have sex at all, but with the understanding that cuddling and touching is where it will end before you fall asleep. You can go for a walk and hold hands, dress up and go for a romantic dinner. Go to a lingerie store and try on some things for him. Whatever you both think of to do that will bring you closer. After a time I would just let it happen.
This could take a very long time, but with the relief of not having other partners to think about and with the concentration on just each other in a way that is loving and romantic, it could turn around with time and patience.
I think sex holds a lot of pressure for people and it really shouldn't. Somehow there is this notion that if you don't look like a porn star and do it like a porn star that you are useless and undesirable. That is so not true.
I always make a practice of looking at everyone around me and assuming that they have sex. Some people are asexual and don't, but for the purpose of the exercise I assume everyone does... It makes me realize that almost no one looks like a porn star and no one is not worthy enough to have something in their life that is as vital as sex. Under this assumption, we all have sex, we all want sex and we all need sex. Everyone is enough, everyone is sexy and everyone is worthy. I really feel that with time and patience and bringing it back to the two of you, that he will discover this and you will also discover that you are enough.
Do the poly thing later, if it is even important by then. It might not be.
I would also prompt him to get a therapist, it sounds like the things that have happened to him need sorting out and moving past. This could help to no end in your situation.
As to friends. Why can he not have female friends? Most of my friends have almost always been male. I don't want to have sex with them all. I just get a long with men... I don't call this poly... I call them friends. I would say he can fill his boots with pretty ladies if he wishes and it makes him comfortable, just no sex (as defined by the two of you) with them.
Anyone want to be friends on Facebook?
Send me your name via PM