The third, and more likely possibility if just dildo play was involved and the amount was copious, is that the lubricant got whipped to a frothy white consistency during play, and got ejected afterward. Some lubricants, such as crisco, are naturally whitish. Crisco thins to a somewhat liquid consistency with body heat and agitation. Silicone lubricants become frothy and whitish if agitated with water-based fluids. Columnist Dan Savage noted the ability of lube to become frothy with his definition of Santorum (after the senator):
"the frothy mixture of lube and fecal matter that is sometimes the byproduct of anal sex"
Sometimes rectal mucous may be ejected as well. However, this rarely amounts to more than a few drops.
In "bareback" porn, the bottom is often seen squeezing out feshly deposited semen. However, what is ejected is often more than just the semen, but includes the lube used during fucking, and sometimes other fluid that may have been deliberately added to make the shot more dramatic.
