> I am firmly against marijuana legalization. This is partially because of this insanity of the pro-legalization arguments.

this is also just motivated reasoning

The insanity of the fringe pro-legalization arguments has no bearing on whether legalization is a good idea or not.

> When I would see friends/family that started smoking regularly become noticeably less intelligent while pro-legalization proponents would argue there are no negative side-effects

This is also just ripe for cognitive bias which is why we should use science to understand these types of claims.