It stems from the historic core teaching on sexuality in the Christian traditions, and it's derivative from Judaism, which likewise held a rejection of 'homosexual' practice. That's why it radically differs from ancient Greece, because they simply did not adopt ancient Greece's standard sexual ethos, the early church held a view of sexuality relatively in line with its Jewish precedence.

As always with this field, we need to keep in mind that contemporary definitions and understandings of sexuality, and homosexuality in particular, are anachronistic when applied to ancient Greece. "Institutionalised homosexuality" is not really very accurate. "Socially prevalent adult/youth same sex partnerings" would be more accurate. This didn't mean either person thought of themselves as a 'homosexual' or homosexual in identity or orientation though.

On a similar note, at what point did the Catholic Church reject birth control? I imagine it wasn't common enough to cause controversy until fairly recently