-- It was a put-down, for no good reason.
-- It personalized the discussion, for no good reason.
-- It violated the principle that one should expect the best of other people.
-- It was a weak argument, violating the principle that one should never
support a strong argument with a weak one.
In short, it was an ignoble idea, poorly expressed, and I'm sorry.
==================
As for the proposition that "F causes ma, and not vice versa" ...
I *have* considered that proposition. I have weight the evidence
for and against. I find the evidence for the proposition vanishingly
weak, and quite overwhelmed by the evidence against.
If somebody has evidence stronger than concluding force causes accelerations
because "the professor thinks that forces cause accelerations" I'd like to
see it.