A possibility Adrian but if this is the case after injecting he would then surely return to the bedroom? Hay doesn't say whether they share the same bed, kiss, cuddle, and caress or how far they go. It seems strange to me that Hay says he talks about sex all the time but won't discuss it with her. I would have thought she could say something like "Do you find me attractive and if so is there a reason why you can't or don't want to make love to me?" This would give him the opportunity to come out with it (the answer, I mean). If he just shrugs it off, she could then say to him. "Please tell me if the reason is due to your cancer situation?" If he says yes, draw him out on whether this is expected to be temporary or long term as you care for him and will understand but it is only fair that you know. If he still shrugs it off, say you are disappointed he won't give an answer and feel he doesn't sufficiently care about you and your feelings and that not talking about your concerns is no way to further develop a meaningful relationship. The fact that he refuses to discuss it indicates you have already tried. Eventually, you may have to decide if you are prepared to have him just as a friend on his terms temporarily or for the long term.
You know the man and we don't, only the ramifications PCa can have on relationships. Indeed, there have been at least two members on this Forum whose partners have left them because they were affected by PCa. Most partners support their men the best way they can, often researching and posting here whether they have sexual relations or not.