I would argue that Pumping Muscle gained the popularity it has (had?) because it doesn't sit nicely in either the "this is perfectly acceptable" or the "the is not acceptable at all" categories. In one sense, we're fine with it because they were paid for being on camera and they could have got up and left at any moment. But at the same time, we kind of like the idea that maybe the guy wasn't entirely comfortable all the time.
Do I think Peter pushed the limits on what the guys were comfortable with doing? Yes, of course. Do I think Peter probably crossed the line of what the guys were comfortable doing, including while filming a video? Again, yes of course (probably why Peter never shuts up during some of the videos).
People on here are quick to view it as a black or white situation. I view it as a bit more gray. In US, we prohibit law enforcement from entrapping individuals into committing a crime. The simplest case that people know of regarding entrapment is the idea that a police officer holds a gun to you and forces you to commit a crime or he will kill you or arrest you for some made up crime. But there are other versions of entrapment that is also prohibited. One of the first cases the US Supreme Court (Sorrells v. United States (1932)) considered was whether an individual was entrapped to sell alcohol during prohibition after he had been asked to do so multiple times. How many times must a government agent ask you to commit a crime before it becomes entrapment? The courts have never stated an actual number or developed a bright line test, so who knows.
Do I think Peter held an actual or a proverbial gun to the models and forced them to do the videos? No. However, I do think Peter did more of the asking multiple times or other things to encourage the guys to do what he wanted. For example, let's say Peter agreed to "sponsor" one of these guys for one or more fitness competitions in exchange for doing these videos. On the first video, he agrees to pay the guy $1,000 (I don't know the actual amount he paid) to do the first video (no nude). When the guy wants to do the second video, Peter tells him that he can't afford to pay him $1,000 for him to do the same thing he did in the first video because it is a business. So Peter will only pay him $500 to do the same or he'll pay the $1,000 if the guys agrees to do more. Depending on the guy's situation, he may feel that he has to do more for Peter. Do this enough times and Peter is either getting some amazing stuff that we all love or he doesn't film the guy anymore.
TL;DR: I don't think Peter is 100% innocent but I don't think he is 100% guilty either.