When the men's cum gets mixed with the woman's cum.
While Christy was sucking Jack's dick, Jack couldn's stand it anymore so Jack fucked Christy & they both cummed at the same time, making a cosmic milkshake as the cums got mixed together.
Result of a gangbang, when several guys finish in the girl, and its all mixed together.
She loves sex so much, she usually ends up with a cosmic milkshake.