Having read about how normal homosexual relationships were in greco roman culture and history, where and when did the homosexual condemnation begin?

In "History of Sexuality Vol. 2", Michel Foucault explores this topic in great detail. Greco-Roman culture spanned a very large time and in surviving texts we can see changing attitudes toward sex even within that culture. The main takeaway is that views on homosexuality were different from today and different from other periods. Foucault says:

"Now, it seems clear, from a first approach at least, that moral conceptions in Greek and Greco-Roman antiquity were much more oriented toward practices of the self and the question of askesis than toward codifications of conducts and the strict definition of what is permitted and what is forbidden."

Basically, it isn't that specific sexual actions like sodomy were considered bad but it was why you were performing them. Sex was looked at as something that was natural to indulge in but overindulgence would be shameful. Sexual need was compared to the need for food. If you are hungry, you should eat but it is shameful to spend all your time eating. This extended beyond homosexual acts:

"Diogenes Laertius reports that in fact he was in the habit of "doing everything in public, the works of Demeter and Aphrodite alike," reasoning as follows: "If breakfast be not absurd, neither is it absurd to breakfast in the market-place."

Socrates focused on the need as well:

"...Socrates, according to Xenophon, gave to his disciples. For if he recommended that those who were insufficiently fortified against the pleasures of love flee from the sight of beautiful boys, and go into exile if necessary, he did not in any case advocate a total, definitive, and unconditional abstention. The Socratic lesson, at least as Xenophon presents it, was that people should "limit themselves to such indulgence as the soul would reject unless the need of the body were pressing, and such as would do no harm when the need was there"

A lot of Greco-Roman views on sexuality were tied up with their views on Masculinity. For instance, are you playing the role of a woman in a homosexual act? The age difference and status of the parties involved was important. Having a younger man instead of a woman would be fine but two men of the same age would be scandalous hence Socrates recommending you run from beautiful boys if you lack self control.

So while their views are different from today or the middle ages, they still would not have been accepting of homosexual relationships as we view them today.