is this sexual assault?

Linsley
My boyfriend suffers from DID, which is dissociative identity disorder. (Look it up if curious) to sum the disorder up in a nutshell, he has different egos, "alters", that come out during stressful situations. Anyway, he has a particular alter that is angry at me. He hates me, to be quite frank. We've gotten into two physical altercations. One in which he smashed my cellphone against a wall, and the other in which he choked me and punched me in the stomach. I hate this alter. 
This alter hasn't came out in quite a while. My boyfriend and I have been having trouble in the bedroom. I want sex more than him. I am very horny, more often than him. And this poses issues in our relationship. We kind of got into a small argument about it, and then went to sleep! 
Next thing you know my boyfriend was pulling my leg over him (spooning position) and getting ready to put it in. I wasn't even awake, I was still pretty much asleep. He kept going and then he came and just laid down. 
That's not usually like him, so I shook him and kept saying his name and all he said was "no." Didn't even open his eyes or anything. At this point I knew it was the alter. 
I felt disgusted, used. 
When my boyfriend returned to his fucking body, I explained what happened. I asked him to communicate with his alter in his head (people with DID can do this) 
I asked him to ask the alter why he did it. His response was 
"The bitch wanted sex, she got it. And I wanted a nut." 
I'm really tore up about it. And I feel assaulted. But it's the man I love... he can't help his disorder. What do I even do? How do I even heal?