In summary of David Halperin's One Hundred Years of Homosexuality, sexuality was profoundly different then how our current society views it.
Greek and Romans did not have identities based around their sexual practices. You were not labeled homosexual for sleeping with a member of the same sex; the act did not confer social identity like it does today.
Greek and Roman males were far more concerned with the submissive/penetrated role; since masculinity was highly regulated in their culture, the penetrated was an inferior position meant for women, slaves, or male youths. The male citizen in this role was transgressing his virtue.
Relationships with the same-sex were never equitable to male-female bonds of marriage since they could not produce children. Plato as the first philosopher of record to denounce same-sex attraction as unnatural. Marriage was also largely public concern being negotiations between families, and producing heirs was an important part of the agreement.
When Christianity came into Rome, sex became a moral issue, and only procreative activities were justified by the laws of God. Everything else was sodomy that degraded one's morality, nothing more than sinful bodily pleasures that damaged the purity of the soul.
Homosexuality was never really celebrated in Greece and Rome, just nobody made a concern of it. Yet when Christianity became culturally dominant, non-productive sex became sodomy, and homosexuality became immoral.