It's well established that high body fat percentage is correlated with lower sex hormone production. Among other factors, this could be due to decreased bloodflow to the genitals.
It's not unreasonable to think maybe the people eating the junk food were simply fatter.
Another concern is whether they're getting adequate nutrients from other foods. There's a bit of substitution going on when a higher percentage of calories coming from junk food, but it's completely possible to eat McDonald's for lunch every day and still get 100% of your recommended vitamins, minerals and fiber from other meals.
This is just poor reporting.