Slaves should not cum and slaves are not allowed to cum.

A sex slave does not need to cum.
A slave does not need to cum to fulfill any of his sexual duties.
The first thing a slave must give up in order to satisfy his mistress is his entitlement to cumming.

A slave should always want to cum in the presence of his mistress.
His desire to cum for his mistress demonstrates his devotion to her.
He shows his mistress he worships her beauty and superiority over him.
He shows his mistress she has power over him and all his sexual thoughts.
By showing his constant arousal, he is consenting to his mistress sexually at all times. She may do whatever she wishes with him sexually and he allows it, by surrendering his erection to her. He submits by being aroused.

She holds all the cards to his sexual existence. He cannot refuse his mistress. He shows this by constant arousal to her. He is always interested and always in want of her. He cannot reject any advance she makes. He feels sexual pleasure at any contact or attention she chooses to give him. He cannot want her less or reject her for anything she does to ignore him.

She controls him at her whim. She may use him for any purpose that benefits her, knowing that he will not worship her any less, whatever that purpose may be.

She may degrade him and violate his body sexually to exercise her own sexual powers over men and benefit from him that way.
She may use him to satisfy her friends in luxurious ways that she could not ask a normal man to. It may be because it is degrading, or laborious, or painful or all of the aforementioned.
She may tease him and frustrate him as entertainment for his suffering predicament.
She may extract favors from him such as gifts or moneyy.
She can have him wait on her hand and foot.
She can whore him out to men as a fuck slut. She can take his money. She can make him pay tribute for sexual liberties. For instance, she can allow him ten seconds of stroking for a hundred dollars earned as a prostitute.
She may reprogram him sexually to receive pleasure doing certain things for her.
She may condition him to feel discomfort in normal sexual activity. This she may accomplish by physical punishment or psychological torture.