Argument
Martiniman: Keanu brings in the $$ because women want his groin in their mouths.
Rebuttal
This argument requires proving that the sole cause of Y is X, where:
X = women want his groin in their mouths
and Y = Keanu bringing in $$
Regardless of how one defines 'women' here, it is fairly evident that X is not universally true. A proportion of women are sexually uninterested in men. Of those who are, not all of them are attracted to Keanu. Of those who are, a proportion do not enjoy putting human body parts into their mouths.
However, the initial argument was phrased to suggest that this was not a premise meant to apply to all women, but just some, and it is, in fact, true that some women would indeed like Keanu's groin in their mouths, for example if they are cannibals.
So, the argument is presumably that those women are directly responsible for Keanu bringing in the $$. While it may seem that the original poster was not claiming they were exclusively responsible for it, that would mean he agrees that other factors may contribute just as much: such as Keanu's acting talent, directors liking to work with him, and Keanu fans who do not want his groin in their mouths but want to watch his movies nonetheless.
However, it was clear from the rest of the context that this was not representative of his position, and that he did in fact believe that women wanting Keanu's groin in their mouths were not just directly but exclusively responsible for his success.
This would require proving that women who want Keanu's groin in their mouths are responsible for the entirety (or at least the majority) of his movies' considerable box-office takings.
In order for the original argument to hold, the poster thus needs to show that people wanting someone's groin in their mouths definitely leads to that someone bringing in $$. Last I checked, men who receive oral sex are much more likely to lose money in the process than gain it, which I think decisively overturns the argument.