"People are doing [illegal thing] anyways at great risk. By making [thing] legal we reduce the risk. That doesn't mean it's not bad; it just means it's safe"
"[Thing] has been legal now for decades but is still considered bad: isn't it time for that to change?"
In other words, you decouple legality from morality when you want to make a thing legal, then you re-couple them again when you want to make it "moral".
You can see numerous examples of this in action, from abortion to drug use.
We've moved from "safe, legal and rare" to "shout your abortion" I'm not surprised that there's a "shout your suicide" crowd in the mainstream.
The Standard Process of Normalization:
In other words, you decouple legality from morality when you want to make a thing legal, then you re-couple them again when you want to make it "moral".
You can see numerous examples of this in action, from abortion to drug use.