Mistress Gaia and her friend wanted to test their new slave. They wanted to gauge what kind of slave he was and how they would punish him. The punishments are usually a good predictor for them and they get to tell with near certainty, the kind of person a slave is. They had this slave lick their feet as well as the soles of their heels before they made him drink pee.
Mistress Gaia has a fetish for slave humiliation and she can use any means she feels like is appropriate to get that done. Today her punishment or humiliation of choice was pee fetish. So she took her slave to her torture chambers and she asked him to drink her pee. He refused and she whipped him painfully and told him to drink her pee again and this time he did what she wanted.