Why do men not cut the hair from their temples? I’ve heard it was because it was a pagan practice, and as such should be avoided. If that’s the case, does that mean that anything that is associated with something pagan is to be avoided?

Yes, anything associated with paganism is to be avoided. The Torah forbids the rounding of the corners of the head. Conjecture has it that the reason is because that was a pagan practice but, as with all of the commandments in the Torah where a reason is not given, we cannot be absolutely certain that we know the true reason.