Non sapient realistic depiction of animal fucking with human is not ok by the rule. Sapience can be easily judged and the rule likely applies only to animals being fucked that appear to either be struggling or show complete indifference and just stand there on all fours
So, this is ok:
Realistic looking animal in cowgirl position fucking with a human dude, since that position requires deliberate effort and willingness
However this is not ok:
Realistic Animal standing on all fours and just taking it without showing any intelligence or having any facial expression other than complete indifference