Having had the experience Ash has had, and being as observant as he was, he knew how to read the signs of when someone was about to orgasm. Increases in pace, decreases in steadiness. Slight temperature fluctuations as blood flow rushed elsewhere. As soon as Ash recognized those signs in Junior, he had a split second decision to make.

His employers strictly forbid their clients to come inside of their synth lovers. Not for any risk of disease or impregnation, of course, but for cleanliness and anonymity purposes. Of course, as soon as he was returned Ash would be thoroughly disinfected before being put into stasis, but if any trace amounts of biological material were found inside his body cavity, he would be docked for it, and Junior would be discontinued as a customer.

Ash decided he didn't want to never see Junior again.

He pulled up, so that by the time Junior came, all of it would spill against their pressed-together skin.