Like any other man, Desmond had all kinds of sexual desires and fantasies, some more common than others. And if there was one that Desmond had in common with many men, it was seeing his partner covered in his seed while she swallowed it. Leaving aside the psychology behind such a fixation, it was as common as the fantasy of having a threesome.
As one might imagine, for Desmond, experiencing both at the same time was like reaching nirvana. Seeing Claire's curious expression as she reached out her hand to wipe some of the white liquid from Sasha's face before putting it on her tongue nearly sent Desmond to an early grave.
That's where fantasy meets reality, as Claire's face immediately showed that she didn't like the taste of what she had just put in her mouth. The same went for Sasha, who was still choking as she tried to wipe the white liquid from her mouth.