The idea of Roman culture being tolerant is based on our own cultural assumptions about the nature of sexuality.

Sex between Roman men was heavily gendered, with the masculine penetrative role being the dominant and socially acceptable one.

To be penetrated was to be feminised, unacceptable for a Roman man, so the only acceptable relationships had a heavy power differential, of age, social class or slave status.

We conceive of sexuality as being attraction to the same or opposite gender, but for the Romans it was about the active, masculine, role and the passive feminine role.

A Patrician being penetrated would be looked upon as deviant, not because he was having sex with a man but because he was having sex in a feminine role.

Someone else will be better qualified to talk about the role of Christianity and the Biblical injunction that 'Man shall not lay with Man' in the shift in worldview.