I was fortunate to be off the hiway, in a track situation, with at least a denim, when I learned this lesson. I have NEVER ridden without leather or helmets since.
The sport bike rear seats appear to me to be a driver's ego trip, in any case. At speeds above 40 MPH, there is simply nothing for the passenger to hang onto unless it's the rider. While I fully understand the '60s mentality of the guy driver and scared girl passenger, the high road I took after my first crash was to always warn her (whoever "her" was) that she would have to consider that she was going to have to hold onto me snugly if she really wanted to experience the Four that day, and give her the chance to back out first. Then, I'd make her wear a helmet, and I carried an extra jacket in the Vetter. And, she could not ride with me "for real" unless she had at least pants and shoes.
Oddly,...in the crowds where I was, I got more passengers. They would wait unti the next weekend after my "comments", but they would show up, ready to ride 'the course' we all shared, for real. That was fun! I still remember one girl in particular, who must have been a weightlifter or something, who kept my under 110 by inadvertently squeezing all the breath out of me every time the needle went above it...