Different providers have different motivations, right?

Honestly, I think some providers really are very much into the sex. Perhaps not with every client, but with the right client and when the right mood strikes, yes. If the stars all align, that can be downright exciting and pleasurable for the provider. Now, some providers will fake being horny -- some might do a better job at faking than others -- and certainly, that's part of what most people consider GFE, right?

But with trans providers who can get hard themselves, there is that extra element. If you were totally uninterested in sex, not attracted at all to the person in the room with you, would you be able to get hard and stay hard, and be able to cum? Some people have that kind of control over their body, I suppose. Not me.  

Are there providers who are thinking about getting laid as they're posting their ad? Yes. Are there other providers who are just going through a routine? Also yes. I think it's likely that the same provider  might feel both of those emotions, depending on the day or her mood.