Trigger phrases are something like: "The {Subject_1} is on all fours."

"{Subject_2} is at {Subject_1 Pronoun} rear pressing {Subject_2 pronoun} crotch against {Subject_1}"

This model can make some funny combos!

This is actually not particularly good at the sex part. I debated calling this "From Behind", but someone might be able to figure it out.

Things to avoid (maybe):

Saying "behind" means in relationship with the viewer, So the {subject_1} could be viewed from the side, and the {subject_2} is behind them, meaning to their far side and not behind the person's rear. Maybe that's desirable for you. I have a statue example that shows what I mean.