It's possible that arousal works like this: we have various theories that trigger various chemical releases, and then other theories about what to do in the sensory environment created by those chemicals. In this case, if the first set of theories (that trigger chemicals) were messed up, we could bypass them with drugs. If the problem was in the second set of theories, about what to do with the chemicals released already, drugs wouldn't help. (You could string in more layers if you liked, like 3 different chemicals triggering in a row, some mechanically, some based on theories)
It's possible that arousal is purely mental without intermediate chemical stuff (or perhaps not according to modern biology, I just mean abstractly conceivable). In this case, drugs wouldn't help, except with physical malfunctions, like messed up blood flow to important bits.
What's not conceivable is to take theories out of it, and declare that "when in a sexual situation" or some such, then chemicals control behavior. For one thing, how do the chemicals know what a sexual situation is? And for another, chemicals controlling behavior is absurd. And for another, people have been known to stop having sex in the middle. (Oh, sorry, the chemicals take *partial* control, which means, ummmm .... nothing coherent).