It had nothing to do with sexuality, but with power and humiliation. Your very words spell it out: it was humiliating for a man to be penetrated, and Jesus was a condemned prisoner, ripe for abuse and humiliation.

Rape usually isn’t about sex. A number of years ago a group of police officers were accused of sodomizing a prisoner with a broom handle. Though they used an object rather than their own sex organs, the principal was the same, and it was no less a rape.