This was inspired by this question. The topic as such is not necessarily opinion based and could be answered by citing scientific studies. But the author already has a clear opinion on the matter and is just looking for confirmation. I voted to close as opinion-based but I'm not sure whether this is a) the correct reason to close and b) whether this should be closed at all? As of writing this question had a negative vote count but so far no close votes.
I think it is fine to ask about arguments in favor of the (proposed) legalization of certain narcotics.
In your example, the question contains some of the author's opinion which in turn attracted down and close votes. That's fine, and it is up to members of the community to cast such votes.
Another option might be to edit out some of the opinionated parts. As long as the author agrees to that (i.e. the edits aren't reverted leading to an edit war) then that's fine as well.
If the author stats reverting such edits because they don't agree with them, then it might be better to take a step back. That doesn't happen often, but when it does it could be cause for voting (down or to close) or flagging it for moderator attention (if the edit war persists).
So in my view, it's up to the community to decide what to do with the question. I don't see a reason to mod-hammer it one way or another.
Given how strongly they went with marijuana = dangerous, I'd say that was a push/loaded question... to rephrase for emphasis, it was something like: how could they possibly legalize something so dangerous? There were some clear signs of that in the q like this kind of alarmism:
It may undermine the nation's gene pool.
There's a more specific close reason for that: "The primary purpose of this question appears to be to promote or discredit a specific political cause..."
I'd say that the top-voted answer did a fair job of debunking some of those preconceptions, thus it worked as a decent frame challenge to the question's main assumption, but it would have been an answer better suited to Medical Science SE and to a straightforward [rather than loaded] q like: "are there [medical] science-based reasons to reduce restrictions on marijuana?"
When 95% of the answer has to be a frame challenge on (medical science) facts... the question is a bad fit here (on Politics SE) for that reason also. As discussed elsewhere, not everything that political decisions may hinge on are on-topic here on Politics SE.