Think back on expositio. According to our sources, most abandoned children died — but some were “rescued,” almost inevitably into slavery. And the most profitable way for a small child slave to earn money was as a sex slave. Brothels specializing in child sex slaves, particularly boys, were established, legal, and thriving businesses in ancient Rome. One source reports that sex with castrated boys was regarded as a particular delicacy, and that foundlings were castrated as infants for that purpose.
Of course, the rich didn’t have to bother with brothels — they had all the rights to abuse their slaves (and even their children) as they pleased. And, again, this was perfectly licit. When Suetonius condemns Tiberius because he “taught children of the most tender years, whom he called his little fishes, to play between his legs while he was in his bath” and “those who had not yet been weaned, but were strong and hearty, he set at fellatio,” he is not writing with shock and horror; instead, he is essentially mocking the emperor for his lack of self-restraint and enjoying too much of a good thing.
This is the world into which Christianity came, condemning abortion and infanticide as loudly and as early as it could.