Typically when we say that some course of action is unthinkable, we have already thought it. Perhaps there is an equivocation, though. Maybe what we mean is that the course of action is one that we can think about with theoretical reason, but cannot deliberate about with practical reason. But isn't the revulsion from the course of action in fact a matter of the will? Maybe it is a matter of the will, but not of the will in respect of deliberation?
I've seen a classic moral theology textbook say that if you've deliberated whether to commit a mortal sin, you've already committed a mortal sin. Of course, it is presupposed here that you're aware that the action you're deliberating about is a mortal sin. Maybe the idea is this. Deliberation involves weighing the pros and cons of an action. But as soon as you've realized that a course of action involves you in mortal sin, it is illegitimate to weigh the pros of the course of action. If you do so, you're implicitly conditionally attaching your will to the mortal sin, conditionally on the pros being great enough. It would be a mortal sin to explicitly conditionally attach your will to a mortal sin--for instance, to resolve to commit adultery if you win the lottery. Whether the implicit conditional attachment in deliberation yields mortal sin is something I am not so sure about. But it is surely sinful.
Of course this needs to be distinguished from a non-deliberative consideration of the benefits of a gravely wrong action. There can be good reason to engage in such consideration. For instance, one might try to think through the benefits of a sinful action in order to come up with a rhetorically powerful exhortation against the action, by contrasting the induced decay of soul with the temporariness of the benefits.