According to people in the 1850s, allowing a woman to write a letter and then post it without anyone in her house ever knowing was going to throw the whole world into chaos. Women obviously could not be trusted to contact only respectable people, and once they started sending letters unchecked, who knows what kind of things they would get involved with (sex things). Even Trollope, the guy who pioneered the pillar box revolution, regretted it the second he realized it was going to give women a tiny bit of (probably sexy) freedom.
But this was nothing compared with the utter destruction of society that the post office was going to bring to America. In Britain, even if you managed to mail a letter in secret, the reply would be delivered to your house and you could be found out. But women in America picked up their letters at the post office, even renting boxes there just like we can today. Obviously, this meant they were all going to become prostitutes.
Women find sorting machines unbelievably erotic.
According to one pseudonymous writer at the time, having "clandestine correspondence with unprincipled men" was already affecting "a thousand schoolgirls a week" and opening their minds to "abnormal channels." He claimed madams had started hanging out in post offices specifically to lure these ready and willing stamp-sluts into a life of prostitution.