Because God gave us two heads, unfortunately only enough blood to operate one at a time, so.. with the penis hard, there isn't enough blood going to the brain to make good choices.. the saying goes, a hard dick has no conscious... and that's exactly true because no blood in getting to the brain
because their significant others shut them off...women use sex to get men, once they've got the man, they think it's OK to shut him off and make him live by their standards