Marijuana no more threatens our culture than does alcohol.
People should not be forced to accept unhelpful changes
Legalizing a non-rights-violating activity doesn't force anything on anyone - and helps by cutting into criminal profits.
The argument can be made that abortion does not threaten a culture. That it is only an individual choice.
I almost wish we could conduct experiments, Culture A without abortion and Culture A with abortion, and then observe the differences over time.
I do believe they would be different.
I can think of differences that are reasonable to expect once fertility is controlled. I would predict a lessening in the valuation placed upon marriage. I would see a callousness develop toward human life.
Those off the top of my head. Can you see any more?