I'd challenge that it was socially acceptable. Consider Suetonius on the subject, who places Nero's marriage to Sporus among his crimes between his raping a vestal virgin and a widespread belief that he desired incest with his own mother!

If the marriage to Sporus did indeed happen - and on the subject of overthrown Roman Emperors one must always be wary of propaganda - it was not because it was seen as acceptable. It was something Nero could do not because Romans as a rule approved, but because he was the emperor, and no one who disapproved was in a position to do anything about it at the time.

when the marriage couldn't have given him any heirs?

About that part, it was common for emperors to adopt as a son the adult person they want to see as their heir. So an heir did not have to be a natural child.

Because he was a Julio-Claudian emperor. I would be careful about giving too much credence to the claim though-Nero was heavily vilified after the fact to justify a change in dynasty from the Julio-Claudians. Never again would a family be able to hold onto power just because they were descended from a particularly respected man (Octavian). So the rebelling generals needed to make him look REALLY unworthy to justify their displacement of the family of Augustus with anyone else.