Really, only the woman has to be in shape. The back seat of a Ford Focus was okay for the ex-nearly-300-pound man.
The studies I’ve found say that over 75% of high school students over 15 yo are driving (closer to 90% in some states), which even if it’s lower than previous years is still a lot of high school students driving. In addition other studies show that more students are driving to school itself than in previous years. I doubt this explains a precipitous reduction in back seat sex if that is in fact itself an accurate observation.