I think it's like a lot of things in porn; the cameraman/director/editor have discretion, and plenty of stuff gets cut because it's "not worthy" of being included in the finished product. Creampie is sort of a niche/novelty today, but it very well could have been throwaway garbage footage, back in earlier days. For example there was a fair amount of natural gaping in anal scenes of the 1980's, but the camera didn't zoom or dwell on it, it was treated as just an incidental thing that happened.

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