Hi O,
There are many reasons they can be against it.
One which I know is going on is siblings are getting their brothers and sisters friend’s numbers from each others Address list and texting and flirting, and, eventually meeting.
They start out anonymous to feel the other out. And being texting is private, many people go further than they would in public.
In my town, they follow up with meeting at the pizza parlor, 7-11, or lake. And it usually ruins their lives for a while.
This thing is WAY more rampant than adults realize.