Look at the fresco from Pompeii - she's at her leisure, naked, but he's wearing clothes. I think it's a dead cert he's her slave.
And, guess what? I found some interesting quotes:
Is your slave's prick the only true one?
--Martial (Roman poet)
Martial also gives a hint that even in mixed baths there could be "women's recesses" (7.35.7: feminei recessus) of uncertain function
--Fagan “Bathing in Public in the Roman World
---Younger “Sex in the Ancient World”
A small percentage of Roman ladies could do pretty much as they liked. Some of what went on must have resembled my darkest femdom fantasies.
Look at this slave. Is he taking breakfast to his mistress? What else will she want? And, under that kilt, is he sporting a theca?
I don't envy him his life. But I'd love to swap places for a month or so...