Maybe it's just me but I'm seeing this from a very different angle. First off, the cheating didn't seem to hurt him that much from your point of view which could point to all number of different things but it's hard to tell. Steroid use can have terrible side effects and it often results in shrinking of the genitals. While you and him may be comfortable with each other, I know a lot of guys who worry about their penis size (me included). While this may not be his main reason, it could be one of many. It may just be a combination of many factors that are leading too it.

One thing is that you should have to make him have sex. Sadly thought, if he says no, the answer should be no. Although you may want it, if he doesn't then it probably shouldn't happen. I've seen people who lose their sex drive because their partner became demanding sexually and it made them feel like they were forced to have sex to keep up, which in turn, made it feel cheap and no longer special for them. I'm not saying that's the reason, but it may be making it harder on him to recover if he feels rushed.

If he is seeing a sex therapist then, despite your urges, it might be best leaving him be for a while and focusing more on the emotional aspect of sex. People often say how their relationship is perfect, save for the area of sex but let's face it, it's important and it failing can have much deeper roots in the relationship than you think. It sounds like you should have a talk with him about everything and try to work through both of your feelings.