Hegel's not a an accurate description of how history works.
Okay, it's the 21st Century, and we're all rational adults. There's no reason to offer up warmed up Hegel as a descriptor of how cultures evolved.
Speaking of the Classical world and Christianity as two opposed and contesting thesis is pretty... not at all true. A good start is realizing that Late Antiquity's Christianity has mores that evolved out of the Classical world. This is the case of an evolution, not a struggle.
The Romans and the Greeks had rigid ideas of sexual propriety, as has been addressed above, and the big thing is not being a passive participant. Homo and hetero sexual relationships are viewed equally, in the sense that both males and females of lower status could be freely used by those of higher status, with the only bar being if the person in question belonged, chattel or patronage, to someone of your status or higher. Submitting sexually, as a male Roman, to anyone not of a higher status, was considered unspeakable.
Port that forward, and you have Christian sexual mores. The evolution here is that Christianity has some idea that everyone has some essential spark that's not determined by your status in this world, and the Gospels and some of the epistles of Paul are shockingly friendly towards the idea of women having this spark too. However, as the early church was very much a product of the society it evolved in, the same ideals of status and hierarchy govern sexual norms of the day to day, and did so for a long time come. Any seeds those doctrines planted would be a long time in flowering. If everyone's equal in the eyes of God, well, then there's no natural inferior to do the catching in a same sex relationship.
The short version? The sexual mores of the classical world largely survived. They're present in every man who excuses their sexual assault as not knowing the victim had a husband/boyfriend, or as being excused by the victims lack of property. Or every male who says they can't be a homo, because they were the active party. It's just the classical world may not have been as idyllic as old episodes of Cosmos had you believe.