>>104233525PG-13 literally didn't exist as a rating until 1984, which is when both those movies came out. It only came into affect in July of that year, so while it was technically in force when Terminator was released, it makes sense that they hadn't really started applying it to movies at the time.
Also R was never used for pornographic films, not even close. Those films were always rated one step above R, as X (now NC-17). The difference between R and X is that you can still admit underage people to an R movie as long as they are accompanied by a guardian, but X rated films do not permit anyone underage under any circumstances.
