Isn't the "dopamine treadmill" a proven effect? As t->infinity with constant gooning, the odds you'll see some gay shit or borderline gay (pegging, shemales, all of the excessive focus on ass stuff) increase. All it takes is for that to do it for you once if you need stronger and stronger stuff to get off. Plus, PIED is definitely a thing, and guys have been known to resort to putting things up their asses to induce an orgasm when the old-fashioned way doesn't do it for them anymore.